What is an Idiom?  We use them everyday, throughout the day.  They are expressions that we do not take literally.  They all have there own meaning, and we learn them as we develop our langauge skills. 

Here are some examples:

It's raining cats and dogs!

Hold your horses!

I put mt foot in my mouth.

I have butterflies in my stomach.

Green thumb.

Seeing the world through rose colored glasses.

Hit the books.

All tied up.

I'm on top of the world.

You got up on the wrong side of the bed.

It goes in one ear and out the other.

Down in the dumps.

Your in the clouds.

I'm all ears.

I let the cat of the bag.