New bitcoins are "mined" or generated by computers.
They get harder to generate all the time, which means the inflow of fresh bitcoins keeps falling.
There are about 8 million bitcoins in circulation today, and the maximum that can be generated is 21 million.
By 2032, 99 percent of those will have been created.
Image: In this April 3, 2013 photo, Mike Caldwell, a 35-year-old software engineer, holds a 25 Bitcoin token at his shop.