I was over the moon

I am 9 years old and need to know the meaning and origin of I was over the moon for my homework. Please help. Thank you

Daniel. D

When someone uses the expression "I was over the moon" it means that person is very excited, overjoyed or happy about something.
I do not know the origin of your expression but I can guess that maybe it has to do with the fact that if you are over the moon you are that much closer to being in the most happiest place of all--heaven!