You've been floating along in the cool water of the lake for a while, and even though you were hot before you dove in, you feel your legs start to shake. You get out and wrap yourself in your towel, and your teeth start chattering. What's going on? You're probably shivering.
Shivers are reflexes (say: REE-fleks-iz), which are things your body does automatically to keep you safe and healthy. Reflexes are controlled by your nervous system, which is made up of your brain, your spinal cord, and lots and lots of little nerves that stretch out all over your body. Nerves are like little strings or wires that carry information.
What kind of information? Well, your nerves sense that the cool water of the lake has lowered your body temperature. Your body needs to stay at a toasty 98.6°F (37°C) for you to be safe and comfortable. The nerves send signals saying, "I'm cold! Warm me up!"
Muscles in Action
That's when things really start to get interesting. The signals go to your brain (telling you to wrap in the towel) and to your spinal cord, which sends a message to other nerves all over your body. What happens next? Your muscles tighten and loosen really fast. Why? They're trying to warm you up, just like taking a jog around the lake would do. Once you get all snug and cozy in your towel and your body warms back up, your brain and nerves tell your muscles to stop shivering.
There are other times when you might shiver, too. Sometimes you'll shiver when you're excited or afraid. When you feel these things, your brain and nerves send out messages through your body that cause your muscles to get excited, so you shiver.
You might notice that when you shiver, tiny bumps form all over your skin. Goosebumps happen because your skin is covered with hair. When the muscles that are attached to each hair get tight, they pull the hair and your skin up into the air.
We call them goosebumps because they look like the skin of a goose or a chicken. So don't be a silly goose the next time you get the goosebumps after swimming. Towel off and put on some dry clothes to warm yourself up!