a hot potato Meaning: a controversial issue or situation that is awkward or unpleasant to deal with. Example: The subject of bullying and fighting in my school is a hot potato. piece of cake Meaning: something that is easy to do Example: Learning English is a piece of cake as long as you do it with our website. once in a blue moon Meaning: very rarely Example: I go to visit my grandfather only once in a blue moon; he lives in a remote farm house. a bed of roses Meaning: easy option Example: Taking care of my younger sister is no bed of roses; she is very silly. raining cats and dogs Meaning: raining very heavily Example: I wanted to go to play outside, but it was raining cats and dogs yesterday. when pigs fly Meaning: something that will never happen or is impossible. Example: William will keep quiet only when pigs fly. devil's advocate Meaning: one who presents a counter argument Example: Hey Jack! You're always playing devil's advocate! Give it a rest and mind your own business. miss the boat Meaning: miss the chance. Example: Peter wanted to enter the drawing competition, but he was too late to enter, and he missed the boat. apple of eye Meaning: someone very precious or dear Example: Every kid in the world is the apple of their parents' eye(s). zip your lip Meaning: to stop talking Example: I don't want to hear another sound out of you. Now do as you're told and zip your lip. Source: theidioms.com