There's something about caves --

a shadowy opening in a limestone cliff that draws you in.

As you pass through the portal between light and dark,

you enter a subterranean world --

a place of perpetual

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Thirteen point eight billion years ago,

the universe as we know it began with a big bang,

and everything that we know and are and are made of was created.

Fifty thousand years ago,

our brains underwen

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Some of the world's most recognizable symbols exist to sell products,

others to steer traffic

or advance political causes.

But there's one whose main purpose is to help people.

You may know it as the

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In the 16th century, the mathematician Robert Recorde

wrote a book called "The Whetstone of Witte"

to teach English students algebra.

But he was getting tired of writing the words "is equal to" over

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