How do you pants someone without them knowing?

If this particular person lives with u take a strong thin wire or string and take a pin or needle and punch a hole in ther pants while there sleeping and put the string or wire threw there pants and tie it goood, after that loosen there belt, then when he/she wakes up sneek up behind him/she and pull the string or wire.if ther person has sweat pants or any pants that dont require belts u wont need to loosen any belt.PLZ make shore this is done in the back of the persons pants.now if its a person that doent live with u or anything i dont really know how to do that.