Muscle is a soft tissue below the skin that can change its length. By doing this it can cause the skeleton to move. Muscles can only contract, then relax. They cannot expand. So they can pull, but not push. So every joint in the body needs two lots of muscles, one on each side. When one set relax and the others contract, the joint moves one way, and to move the other way the opposite happens.

Muscle cells contain protein filaments that slide past one another, producing a contraction. Muscles also wrap around our torso to hold in our vital organs. Some vital organs are entirely muscle. For example the heart is the most important muscle in the body. Other muscles squeeze food down the guts.

Muscles need energy to work and this comes from the fats and carbohydrates we eat.

Muscles need to be worked to stay healthy. They start to wither away when they are not used. This is why it is important to exercise, not necessarily by working in a gym, but by doing anything that gets you warm or makes you sweat. Funnily enough, muscles will last longer (and so you stand a chance of living longer) if you keep exercising them.