Remember how your kidneys filter out the waste and make pee?

Sometimes there is too much of certain stuff—like calcium—and the kidneys cannot remove it all. That extra stuff can stick together and form tiny crystals.

The crystals build up, like when you press little pieces of modeling clay into one big ball. That is a kidney stone.

Kidney stones can be very painful, cause bleeding, and block pee, but doctors know how to remove these little troublemakers.