You hold your hand to your chest and feel your heart go
"thump, thump, thump...". What is a heart? It's really just a muscle, like those in your arms and legs.
Located a little left of the center of your chest, the heart is about the size of your fist. The heart's job is to pump blood around to different parts of the body. It sends oxygen and nutrients out, and carries back waste.
The two sides of the heart each have their function. The left side brings in oxygen-filled blood from the lungs and delivers it to the body, while the right side receives circulated blood from the body.
The "thump" you feel is the heart filling with blood, then squeezing the blood out. Your heart does this all day along, all by itself!