Piece of cake

e.g. That exam was a piece of cake! = That exam was really easy!

Costs an arm and a leg

e.g. It costs an arm and a leg. = It is very expensive.

Let the cat out of the bag

e.g. Jenny let the cat out of the bag about John’s surprise birthday party = Jenny told the secret abou…

Hit the nail on the head

e.g. You hit the nail on the head! = That’s exactly what I meant to say!