I always appreciate the double twist. Let 'em think they know what's going to happen and then totally blow their minds. No Way Out was a great movie example.
I once did a demonstration for a bunch of 12 year-olds. I had three glasses of clear liquid and a box of baking soda. As soon as I unpacked the soda a kid in the front row said, "I know what's going to happen. I know what's going to happen." And kept it up. So when I stirred a spoonful of the baking soda into the first glass and it turned GREEN he was the most surprised of the bunch. Then when the second glass foamed over even he appreciated the trick.
