¹     Tickle your little brother. Read a book. Skate. Smile. Each time you do one of these things, your muscles are at work. Each time any part of you, big or small, moves, you use your muscles.
 
²     You have over 600 muscles in your body. Some are big. Some are small. Some are right under your skin. Some are deep inside your body.
 
³     Make a fist and bring it up to your shoulder. You can feel the biceps, a muscle in your upper arm change as it bunches up and gets harder. How did that work? Well, the muscle is attached to a bone. As the muscle contracts, drawing itself together, it pulls on your arm. It works with another muscle called the triceps. The biceps flexes, or contracts, to pull the lower part of the arm up toward the shoulder. At the same time, the triceps extends, or stretches, on the back side of the upper arm. Other muscles in the body are paired the same way.

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