Our body can do so many things. Inside the body are bones. Bones are hard and give the body its shape, like the frame of a house. Where two bones meet there is a joint, such as the elbow, the knee, the wrist, and the ankle. Joints let us bend, twist, and turn.
Muscles are attached to bones. Muscles pull on bones to make us move. With bones, joints, and muscles we can walk, run, jump, climb, dance, throw a ball, and write our name. Inside the chest the heart beats all day and night, and the lungs move in and out as we breathe.
Example: Put your hand on your chest and feel your heart go thump-thump. Now bend your elbow, then your knee. Those are joints at work! Jump ten times — your muscles did that.