Sit as still as you can!
Even when you think you are completely still some of your muscles are still moving.
Your heart is beating and your intestines and lungs are working.
All these movements are made by muscles. You can't control these muscles; they will keep on working whether you like it or not.
Many muscles are attached to bones but some are not. The muscles that move your lips, eyelids, eyebrows and eyes are not attached to bones.
When you smile, frown or look sad you are using muscles that are under the skin on your face.
It takes about 15 muscles to make you smile, much less than it takes to frown.
More about muscles