Can you explain the human skeletal system to me?

OK, well, apparently there are no similar questions to mine so here goes.
I know babies have 300 bones in their bodies, and adults have 206. Why does this happen/Why is this so? Also, does this occur in other animals (less bones when they reach adulthood) such as dogs and cats and rabbits?

HINT HINT 10 points await the person who answers me best. Deadline: ASAP, or whenever question expires