Bundles inside bundles
A muscle is made of long, thin cells called fibers. Each fiber is thinner than a hair.
Fibers are packed together in bundles, like a pack of uncooked spaghetti. Then those bundles are wrapped together into bigger bundles. The whole thing is a little like a rope made from lots of strings.
Inside each fiber are even tinier threads. When the muscle works, the threads slide over each other and the fiber gets shorter.
Try it: lace your fingers together loosely, then slide them in deeper. Your hands got closer together. That's a bit like how the tiny threads slide.
Pull, never push
When a muscle works, it gets shorter and fatter. It pulls on the bone it's tied to, and the bone moves.
When the muscle relaxes, it goes back to being long and soft. But a muscle can't push. It can only pull.
So muscles work in pairs. One pulls a bone one way, and its partner pulls it back. Your biceps bends your elbow, and your triceps straightens it.
Try it: put your hand on your upper arm and bend your elbow. Feel it get hard? That's your biceps pulling.
Your brain is the boss
Say you decide to wave. Your brain sends a message down your nerves to your arm muscles, a bit like a phone call down a long wire.
The message tells the muscle fibers to squeeze. It all happens so fast that you don't notice any wait.
When you learn something new, like riding a bike, your brain and muscles practice together. It gets easier every time. People call this 'muscle memory,' but the memory is really kept in your brain.
Stronger with use
When you run, climb, swim and play, your muscles work hard.
Afterward, your body fixes them up and makes them a little stronger. Muscles that hardly get used slowly get weaker.
Rest and sleep are when a lot of that fixing happens. Food and water give your muscles the energy to work.
Legs feel tired after a long hike? That's normal. A good night's sleep helps them get ready for tomorrow.
Warm up first
Cold muscles are a bit stiff, like modeling clay that's been in the fridge.
Moving gently first, like marching in place or swinging your arms, gets your blood flowing and warms your muscles up.
Warm muscles stretch more easily. After you play, slow down with an easy walk. That's called a cool-down.
Muscles make heat
Every time a muscle works, it makes some heat. That's why you get hot when you run around.
When you're cold, your muscles shiver. They make lots of tiny, quick squeezes all by themselves, and that makes heat to warm you up.
Try it: do 20 jumping jacks, then touch your cheeks. Feel warmer? Your busy muscles made that heat.