Kidney Stones
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 pee, and block pee from coming out of the kidney.