Idioms are used for another way of saying things, in a more, creative way. As an example, if you were to say "That test was easy." You could have said it "That test was a piece of cake."
Get off your high horse
Tongue in cheek
Spill the beans
Cat got your tongue
Wet blanket
Knock your socks off
For the birds
All thumbs
Jump the gun
Driving me up the wall
Bad Blood
A good...