My words are an "excuse me" with that glare that says tell me I just did not see/hear what I think I did. Works for almost anything around here. When I say it they know by my tone of voice and degree of glare how much trouble they are in. Often they look back at me and do the right thing like give a toy back, pick up what they dumped, etc. For food issues I limit what goes on the plates but also don't believe in teaching a child that the tray is a plate. From the beginning the cherrios go on a plastic lid from a container - something unbreakable when tossed but to get the point across that the plate is not a toy and stays put.