Kids ask a lot of questions.

mental_floss has answers. This week we launched WHY?, our new series for kids and parents. We'll tackle all types of questions children have about how the world works by providing science-based, kid-friendly content. Our answers are written with early readers (ages 4 to 7) in mind, but we think they're interesting—and educational—for everyone.

Have a question? Send it to why@mentalfloss.com.

A bruise, or contusion (con-TOO-zhun), is one way your body shows you’ve been hurt. If you bang your leg or arm hard enough, the smallest blood vessels in your body, called capillaries, can break. Blood leaks out of the capillaries and makes a puddle in your skin layers. Bruises can look black, purple, or blue at first, and later fade to green, yellow, or brown. 

Your blood vessels have an important job; a bruise shows that there’s been a problem getting the job done! Blood vessels are part of your circulatory (SIR-kyool-uh-tor-ee) system, which moves blood from your heart throughout your body, and then brings it back to your heart again. Capillaries are the smallest blood vessels, arteries are the biggest, and veins are in between. They work together like a highway system, moving blood everywhere from your head to your toes. 

Blood is filled with stuff your body needs, like oxygen, water, chemicals, nutrients, and antibodies, which are the cells that help keep you from getting sick. A bruise shows where there’s a break bringing that important stuff through your body. But guess what: Your body fixes it right away! When capillaries break and leak blood, they squeeze themselves smaller to stop any more blood from escaping. The blood from the bruise is slowly broken down, or metabolized. That’s when your bruise changes color. In about two weeks, the bruise will disappear.   

For fun further reading, visit the KidsHealth page on bruises