Here is a great trick:
How to cut an orange without a knife .
Give your mark a cigarette, lighter and an orange. Tell him to just cut up the orange using only the lighter and / or cigarette.

Solution:
Remove the filter-tip. Hold it in the flame of the lighter. After a few seconds, remove it from the flame, and press the tip flat with two fingers. The tip will turn hard in a few seconds. So now all you got to do is cut the orange with your little self-made knife. And there you got a cut up orange without using a knife. Simple. ( You can also use a lemon or lime. )