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1.Scratch someones back.
2.When pigs fly.
3.Piece of cake.
4.Hit the nail on the head.
5.Let the cat out of the bag.
6.Break a leg.
7.Hit the books.
8.Costs an arm and a leg.
9.You can't judge a book by its cover.
10.Bite more off more than you can chew.
These are some examples.