This is a great way to reveal a lost card or prediction. Its the CUP! Simply pour a hot beverage into the cup and watch it change from a black cup into a white cup with an image of the seven of spades! This is a very visual way to produce a revelation. And great fun for the office! Safe to use as a normal drinking mug.
http://www.wizardhq.com/servlet/the-14999/cup-by-nanswer/Detail?source=googlebase