Bones are very hard.  Bones give our body its shape.

Our body has a skeleton.  A skeleton is made of the bones in our body.  A skeleton has the shape of our body.

Muscles help us to move.  

Our muscles are made of tissue.  Tissue is soft and can move.  Tissue covers our bones.

Bones and muscles work together to give our body its shape and to help us move.  Something that works together to make something happen is called a system.