You're running around with your friend, laughing your head off, when suddenly you trip over a rock and hit the ground. As you pick yourself up, you notice that your knee is bleeding. But while you're trying to figure out where that rock came from, the blood from the cut on your knee is already busy at work creating a scab.
As soon as you scrape or break the skin anywhere on your body, special blood cells called platelets (say: PLAYT-lits) spring into action. Platelets stick together like glue at the cut, forming a clot. This clot is like a protective bandage over your cut that keeps more blood and other fluids from flowing out. The clot is also full of other blood cells and thread-like stuff called fibrin (say: FY-brin) that help hold the clot together.
So now you're home, you're cleaned up, and you're not bleeding anymore. But things are still happening on your knee. As the clot starts to get hard and dries out, a scab forms. Scabs are usually crusty and dark red or brown. Their job is to protect the cut by keeping germs and other stuff out and giving the skin cells underneath a chance to heal.
If you look at a scab, it probably just looks like a hard, reddish glob. But under its surface, all kinds of things are going on. New skin cells are being made to help repair the torn skin. Damaged blood vessels are being fixed.
White blood cells, the kind that fight infection to keep you from getting sick, go to work by attacking any germs that may have gotten into the cut. White blood cells also get rid of any dead blood and skin cells that may still be hanging around the cut. By the time it's all done, a new layer of skin will have been made.
Eventually, a scab falls off and reveals new skin underneath. This usually happens by itself after a week or two. Even though it may be tough not to pick at a scab, try to leave it alone. If you pick or pull at the scab, you can undo the repair and rip your skin again, which means it'll probably take longer to heal. You may even get a scar. So let that scab sit there — your skin will thank you!