Take a glass of water about 3/4 full. Next, place a piece of glossy cardboard over the top. Hold it. Slowly, turn the glass upside down. Take your hand from the cardboard. It will hold.

Until you get it just so, practice over a sink.

BTW, it is not a trick so much as it is science. The cardboard is not 'stuck' there by an adhesive quality. Gravity holds it there.

PS Tell the kids to do it at home when an adult is there with them. They will make mistakes--and wets messes--until they get it right.