Let the cat out of the bag / Not my cup of tea / Spill the beans / Clear as mud.
Idioms have been on my mind lately. For example, I'd been wondering how actually to define an idiom, and what idioms students really need to know. I also had vague recollections of looking through an Academic English textbook a couple of years ago that insisted students be taught that words such as 'affluent' are idioms, because of the Latin etymology from ad+fluere = flowing towards. While this may be linguistically accurate, it's hard for me to imagine any student being enough of a language nerd to be helped by such an approach. Anyway, isn't all speech necessarily figurative, when we decide that a sequence of sounds represents meaning?