Make a muscle with your upper arm like the yellow guy at right is doing. Place your other hand on top of the bulging muscle (called the bicep) and straighten your arm. Can you feel something moving? What's really going on in there?
Muscles are made up of bundles of muscle fibers, which are long and skinny. Inside these fibers are smaller units, called fibrils, which contain stringy molecules of two kinds of protein: myosin (orange) and actin (blue). When your brain tells the muscle to get shorter, or contract (when you "make a muscle"), the myosin and actin molecules slide along each other. When you stretch, they slide apart and get longer:
Now place your hand on the muscle (tricep) underneath your arm, and straighten and bend your arm again. Can you feel the tricep move? Notice that when the bicep is contracted, the tricep is stretched, and when the bicep is stretched, the tricep is contracted. Many of the muscles in your body work in opposite ways like these two. This helps to keep you balanced.
So, now do a little dance around the room, and imagine all of your muscle fibrils sliding in and out, in and out, in and out... And the next time you eat a piece of meat, look for the muscle fibers!