Your two kidneys (say: KID-nees)
work hard to keep stuff you don't need out of your blood. That waste is combined with
water to make your pee.
But sometimes the stuff in your pee (like calcium) can make little crystals that
build up to become hard stones. Depending on the size, these kidney stones can be
painful, cause blood in the urine, and block pee
from coming out of the kidney.