Here's a short, beautifully shot video: a visual explanation of how the human population got to 7 billion, using tinted water dripping into and out of seven glasses -- one for each continent. Drops that go in the top are births, drops that leak out the bottom are deaths. Using this simple visual explanation, it's fascinating to watch the relative populations of the continents over time. Here's what NPR, the video's publisher, adds:

It was just over two centuries ago that the global population was 1 billion — in 1804. But better medicine and improved agriculture resulted in higher life expectancy for children, dramatically increasing the world population.

As higher standards of living and better health care are reaching more parts of the world, the rates of fertility — and population growth — have started to slow down, though the population will continue to grow for the foreseeable future.

U.N. forecasts suggest the world population could hit a peak of 10.1 billion by 2100 before beginning to decline. But exact numbers are hard to come by — just small variations in fertility rates could mean a population of 15 billion by the end of the century.