Show a large black and white picture of a very cute clown. Also show a large, empty, wooden picture frame. Place the picture in to the frame so the kids can see the clown. Explain that with some help from them, you will magically color in the picture. Turn the picture back to the audience. Have the kid all toss "invisible" colors towards the picture. Then, when you turn it around -- what's this?? It hasn't colored in. Of course not, you haven't said the magic words. The kids all shout the words and, as you remove the picture from the frame, it visibly colors in. That's right, as the picture comes out of the frame, they actually see the colors appear.