Offer to read your victim's fortune. Children cannot resist this. Take their hand, palm upwards, and begin to run the palm with your own palm. Ask a series of questions, as though you are gathering the necessary information for your predictions.
The accepted sequence is
* Do you live in this town?
* Do you live in a house?
* Does it have a kitchen?
* Does it have some stairs?
* Does it have a bathroom?
* Does it have a toilet?
* Do you use toilet paper?
The victim will answer 'yes' to all these questions. Ask them slowly to create an involving and eerie atmosphere. After the last question, simply say "I don't, I use my hands." Another success!
Note: If you are tricked into the victim's role, and you are aware of the procedure, you may effect a daring reversal by answering the last question with "No, I use my hands." Blockered!

Taking this from the opposite angle, simply ask someone fat / ginger / small whether they use their left or right hand to wipe their arse.
When they tell you, simply say "I use toilet paper" and run off to tell everyone, making the the universal "uuuurrrrrrrr" sound of disgust.