Muscles and bones together enable us to move our body parts.

 Fill in the crossword puzzle using these clues.

a.    The pipes in our body through which blood flows.
b.    A sense organ
c.    Messages from all parts of our bodies flow to the brain through them.
d.    The breaking down of food into tiny, simple pieces before it can be absorbed by the body.
e.    Something formed by several organs working together.

The heart is a very delicate organ in your chest. But if you hit your chest, the heart does not get damaged. Why?