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Imagine NASA announced today
that they found aliens.
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Bacteria on Mars, weird alien fish
in the oceans of Europa,
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and also ancient alien ruins on Titan.
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Wouldn't that be great?
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Well, no. It would be horrible news,
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devastating even.
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It could mean that the end of humanity
is almost certain
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and that it might be coming soon.
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Why?
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Why would the most exciting
discovery of our lifetime
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be bad?
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Let us imagine the development of life,
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from its inception to us today,
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as a flight of stairs.
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The first step is dead chemistry
that needs to assemble itself
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into self-replicating patterns,
stable and resilient,
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but also able to change and evolve.
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The second step is for our early life to
become more complex,
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able to build more complicated structures,
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and use the available energy
much more efficiently.
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On the next step, these cells combine
to become multicellular beings,
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enabling unbelievable variety
and further complexity.
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The step above sees the species
evolve big brains,
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enabling the use of tools, culture
and shared knowledge,
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which creates even higher complexity.
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The species can now become the
dominant lifeform on its planet,
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and change it according to its needs.
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First shy attempts to leave its
planet are happening.
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This is where we are now.
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It's in the nature of life as we know it
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to reach out, to cover every
niche it can.
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And since planets have a limited
carrying capacity and lifespan,
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if a species wants to survive,
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it will look for more places
to spread to.
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So the steps above the current
ones seem logical:
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colonize your own solar system,
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then spread further to reach
other stars,
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to the possible final step:
becoming a galaxy-wide civilization.
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It's very likely that this is a universal
principle for civilizations,
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no matter where they're from.
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If a species is competitive and driven
enough to take control over its planet,
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they'll probably not stop there.
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We know that there are up to 500 billion
planets in the Milky Way,
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at least 10 billion Earth-like planets.
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Many have been around billions
of years longer than Earth.
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But we're observing zero
galactic civilizations.
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We should be able to see something...
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...but there's nothing.
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Space seems
to be empty and dead.
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This means something is preventing living
things from climbing the staircase,
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beyond the step we're on right now.
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...Something that makes becoming
a galactic civilization
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extremely hard, maybe impossible.
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This is the Great Filter.
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...A challenge or danger so
hard to overcome,
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that it eliminates almost every
species that encounters it.
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There are two scenarios:
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One means we are incredibly
special and lucky,
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the other one means we are doomed
and practically already dead.
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It depends on where the filter
is on our staircase:
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behind, or ahead of us?
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Scenario 1:
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Scenario 1:
The filter is behind us.
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We are the first.
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If the filter is behind us,
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that means that one of the steps we passed
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is almost impossible to take.
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Which step could it be?
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Is life ITSELF extremely rare?
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It's very hard to make predictions
about how likely it is
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for life to emerge from dead things.
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There is no consensus.
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Some scientists think it develops everywhere
where the conditions are right;
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others think that Earth might
be the
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ONLY living place in the universe.
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Another candidate is the step
of complex animal cells.
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A very specific thing happened
on this step,
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and as far as we know, it happened
exactly once.
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A primitive hunter cell
swallowed another cell,
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but instead of devouring it,
the two cells formed a union.
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The bigger cell provided shelter,
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took care of interacting with the
environment and providing resources,
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while the smaller one used its
new home and free stuff,
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to focus on providing a lot of extra
energy for its host.
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With the abundant energy, the host cell
could grow more than before
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and build new and expensive things
to improve itself,
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while the guest became the
powerhouse of the cell.
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These cells make up every
animal on the planet.
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Maybe there are billions of bacteria-covered planets in the Milky Way,
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but not a single one, apart from us,
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has achieved our level of complexity.
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...or intelligence.
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We humans feel very smart and
sophisticated
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with our crossword puzzles and romantic novels.
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But a big brain, is first and foremost,
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a very expensive evolutionary investment.
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They are fragile,
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they don't help in a fistfight with a bear,
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they cost enormous amounts of energy,
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and despite them, it took modern humans,
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200,000 years to get from sharp sticks
to civilization.
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Being smart does not mean you get
to win automatically.
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Maybe intelligence is just not so great,
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and we're lucky that it worked out for us.
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Scenario 2:
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Scenario 2:
The filter is ahead of us.
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Plenty of others died already.
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A Great Filter before us is orders of magnitude more dangerous
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than anything we encountered so far.
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Even if a major disaster killed most of us or threw us back thousands of years,
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we would survive and recover. And if we can recover,
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even if it takes a million years, then it's not a Great Filter,
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but just a roadblock to an eventual galactic civilization.
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On universal timescales, even millions of years are just the blink of an eye.
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If a Great Filter really lies before us,
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it has to be so dangerous,
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so purely devastating and powerful,
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that it has destroyed most, if not all,
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advanced civilizations in our galaxy
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over billions of years.
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A really daunting and depressing hypothesis
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is that once a species takes control over its planet,
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it's already on the path to self-destruction.
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Technology is a good way to achieve that.
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It needs to be something that's so obvious,
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that virtually everybody discovers it,
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and so dangerous, that its discovery leads almost universally
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to an existential disaster.
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A large-scale nuclear war,
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nanotechnology that gets out of control,
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genetic engineering of the perfect super bug,
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an experiment that lights the whole atmosphere on fire.
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It might be a super-intelligent AI
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that accidentally (or purposely) destroys its creators.
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Or things that we can't even see coming right now.
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Or it's way simpler:
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species competitive enough to take over their planet
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necessarily destroy it while competing with each other for resources.
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Maybe there are runaway chain reactions in every ecosystem
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that once set in motion, are not fixable.
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And so once a civilization is powerful enough to change the composition of its atmosphere,
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they make their planet uninhabitable
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100% of the time.
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Let's hope that that's not the case.
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If the filter IS ahead of us,
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our odds are really bad.
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What we can hope for.
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THIS is why finding life beyond Earth would be horrible.
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The more common life is in the universe,
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and the more advanced and complex it is,
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the more likely it becomes that a filter is in front of us.
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Bacteria would be bad,
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small animals would be worse,
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intelligent life would be alarming.
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Ruins of ancient alien civilizations...
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would be horrible.
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The best case scenario for us right now
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is that Mars is sterile,
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that Europa's oceans are devoid of life,
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and the vast arms of the Milky Way
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harbor only empty oceans hugging dead continents.
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...That there are billions of empty planets waiting to be discovered
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and to be filled up with life.
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Billions of new homes...
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waiting for us...
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to finally arrive.
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How likely is it that we'll find life outside of Earth that is similar to us?
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Well, that depends on how many planets there are out there
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in their star's Goldilocks Zone--
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the area around the star where water can be liquid.
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Because stars come in all sizes and configurations,
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this zone is different for every star system
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and requires a little bit of physics to figure out.
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If that sounds like fun to you, this quiz from Brilliant
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helps to break down the maths for exactly how this is calculated.
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Brilliant is a problem-solving website
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that teaches you to think like a scientist
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by guiding you through problems.
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They take concepts like these break them up, into bite-sized bits
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present clear thinking in each part, and then build back up to an interesting conclusion.
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If you visit brilliant.org/nutshell or click the link in the description,
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you can sign up for free and learn
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all kinds of things.
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And as a bonus for Kurzgesagt viewers,
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the first 688 people will also get 20% of their annual membership.
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And if you DO find life on other planets,
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it may be wise to leave them alone for a while.14719
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