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Speaker 1: Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,

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Speaker 1: it's Christian saga and today's question is what is going

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Speaker 1: on with the birthday paradox. You've probably heard this one before,

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Speaker 1: the idea that if there are twenty people in a room,

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Speaker 1: there's a fifty fifty chance that two of them will

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Speaker 1: have the same birthday. So how can this be? Well,

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Speaker 1: it really is called the birthday paradox, and it turns

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Speaker 1: out it's useful in several different areas, for example, in

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Speaker 1: cartography and hashing algorithms. You can try it yourself the

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Speaker 1: next time you're at a gathering of people, you know,

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Speaker 1: just ask everyone for their birthday. I mean, don't be

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Speaker 1: creepy about it, play cool, say you know, something like

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Speaker 1: I'm trying to prove this for science or whatever. And

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Speaker 1: it's likely that two people in this group will have

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Speaker 1: the same birthday, not around the same time, they will

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Speaker 1: have the exact same day. And this really surprises people.

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Speaker 1: So the reason isn't so surprising. It's because we're used

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Speaker 1: to comparing our particular birthdays with some other individuals particular birthday. So,

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Speaker 1: for example, you meet somebody randomly and you ask her

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Speaker 1: what her birthday is, the chance of the two of

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Speaker 1: you having the same birthday is only one out of

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Speaker 1: three hundred and sixty five, or four point to seven percent.

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Speaker 1: In other words, the probability of any two individuals having

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Speaker 1: the same birthday is low. Even if you asked twenty

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Speaker 1: individual people, the probability is still low. It's less than

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Speaker 1: five percent. It's natural that we feel like it's very

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Speaker 1: rare to meet anybody who has the same birthday as

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Speaker 1: our own. But when you put twenty people in a room, however,

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Speaker 1: the thing that changes is the fact that each of

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Speaker 1: these twenty people is now asking each of the other

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Speaker 1: nineteen people about their birthday simultaneously. Each individual person only

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Speaker 1: has a small chance, less than a five percent chance

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Speaker 1: of success, but everyone's trying at the same time, and

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Speaker 1: that increases the probability dramatically. So the next time you're

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Speaker 1: with a group of twenty or thirty people, why not

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Speaker 1: give it a try. You might be surprised. Check out

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Speaker 1: the Brainsuff channel on YouTube, and for more on this

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Speaker 1: and thousands of other topics, visit how stuff works dot com.