Borrow a hat from a spectator, (or use your own) and show it to be totally empty. Explain that you'd like to show the audience how to bake a cake. Bring out all the ingredients -- eggs, milk, flour, sugar, all kinds of stuff. Mix all the ingredients together in a bowl, then "accidentally" drop all the ingredients into the borrowed hat. Oops. Well, no problem. Just drop a match into the hat to heat up the ingredients and hopefully turn it into a cake. Needless to say, the spectators will be a bit concerned as flames come out of the top of the hat. But, it's magic to the rescue. Say the magic words ("Betty Crocker") and -- voila -- when you turn over the hat, out comes a perfectly baked cake.