Memory Game Idioms 2Online version Players will try to match the idiom with it's meaning. by Lunarn Smith Happens very rarely; uncommon Good idea or invention Tell a secret Once in a blue moon Let the cat out of the bag It’s not rocket science Very expensive Best thing since sliced bread It’s not hard. Wait and be patient Burn the midnight oil Cost an arm and a leg Don’t start problems or cause trouble Hold your horses Work until late at night Let sleeping dogs lie Ants in one’s pants Can’t stay calm (relax) because you’re nervous