but, you know, occasionally you find one. And you look at it,

and you have-- at least I have an immediate question of, hey, if it's noon,

how long will it be-- you know, these are lined up-- how long will it be until

they're lined up again?

Well, not... it's more than an hour, but how long until-- just at exactly what time

will the clock hands line up? Well, okay, a little bit after one

it'll line up. And it'll line up again a little bit after two.

So, what's the time difference between there? You know, you say, well, It's more than an hour,

how can I figure that one out? I'm sitting there, saying yeah, you know, I remember sitting in a

fifth grade classroom trying to figure it out, drawing diagrams and thinking, boy,

I need to add a little bit more, a little bit more,

just, it's gonna be a little bit more than that. It's gonna be...

And then,

you know, it's going to be more than that, more than that. How much time until...?

And then, (chuckles)

like you, I realized, oh, there's a simple way to solve it.

Let's go back to noon.

How many times do these hands cross in 12 hours? Well, there's once,

twice,

three times, and so on.

In fact, think about it, and you'll realize the hands cross each other 11 times

every 12 hours.

Ah! If there's 11 hand crossings,

every 12 hours, that means that they will cross each other. So that means...

How long will it take between here to here?

That must be 12 divided by 11 hours,

which happens to be 1 hour, 5 minutes and 27-something seconds.

My clock is almost out of potential energy, so I might as well wind it up.

Recharge my old Seth Thomas for a week.

- Thanks for watching this numberphile video.

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[clocks chiming and ringing]

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