WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your skin protects you by keeping germs out of your body. But when something hard or sharp breaks through your skin, it makes a cut, or a wound. You start to bleed because skin and blood vessels are cut open. Your body works fast to close up the wound so germs can’t get inside and make you sick. But sometimes the wound itches as it heals.

When you get a cut, special blood cells called platelets start sticking together around the opening of the wound. This is called a clot, and it stops the bleeding. As a clot dries, it turns into a scab. The scab helps to keep germs out. The skin around the wound may get pink and swollen. That’s because your body is fixing broken blood vessels and growing new tissue. That’s when you might begin to feel itchy.

Your body releases a chemical called histamine (HISS-tuh-meen) that helps your cells fight infection, close the wound, and fill it in with new tissue. But histamine causes a kind of allergic reaction that can make you itch. Your skin also has special nerves that get irritated when skin and tissue grow and stretch to fill in a wound. Those irritated nerves send a signal to your brain that you have an itch, and that makes you want to scratch it. But don’t scratch! You could scratch right through the scab and cause your wound to bleed. Then the healing would have to start all over again!

To learn more about how wounds heal, check out this awesome animation at The Kid Should See This. Some of the words will be new to you, so watch it with a grown-up!