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A thousand years ago, there were only a third of a billion people.

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But we were multiplying.

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Now there's seven billion of us.

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How did we grow so much so fast?

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Say this glass is North America.

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The fuller the glass gets, the more people there are.

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Water drips in as people are born.

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Water drips out as people die.

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Okay, here's the entire world.

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Now let's go back a thousand years.

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The Americas are nearly empty.

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Europe and Africa have less than 100 million people between them.

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And just like today, most people live in China, India, and the rest of Asia.

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For centuries, things stay pretty much the same.

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Births are canceled out by deaths.

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Women are having lots of babies, but most babies die before they grow up and have families of their own.

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We don't reach 1 billion until 1804.

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But things are changing.

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Better medicine and better agriculture are starting to slow the leak from the bottom of the glass.

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People still die, of course.

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But more babies grow up and have babies of their own.

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Lots of babies.

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We've grown from 1 billion to 7 billion.

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And it only took 200 years.

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Will we keep going like this?

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Probably not.

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The UN says we'll level out at 10 billion by the end of the century.

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So as we continue to grow from 7 billion,

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the question is, can the world hold that many people?

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Experts can't agree.

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It all depends on how well we manage our food, water, and energy.