Here it is:
1. Piece of cake – No, when someone says that the assignment they just finished was a piece of cake, it does not mean that their professor gave them a red velvet cupcake for their midterm paper, what piece of cake actually means is that something is very easy to complete.
2. Costs an arm and a leg – It would be a strange world we lived in if buying that fancy shiny purse literally required us to chop off our body parts to give as tribute to the Louis Vuitton gods. When something costs an arm and a leg it actually means that something is very expensive.
3. Break a leg – Oh, look, another idiom about legs. You’re about to take your dreaded calculus final and before you head into your classroom your roommate texts you, “Break a Leg!” Why, you think in your head, would he ever wish that upon me? I thought we were cool with each other. Well, your roommate surely doesn’t want your bones to break while walking to your seat in the exam room that’s for sure. Break a leg actually means good luck!