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 urine, and block pee from coming out of the kidney. Back to Articles Related Articles Your Kidneys You need at least one kidney to live. Find out why in this article for kids. Read More Your Urinary System You pee every day, but what makes it happen? Find out in this article for kids about the urinary system. Read More Movie: Urinary System Watch this movie about the urinary system, which produces pee. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.