Knox
Place several packages of "Knox" (clear geletin) in the toilet of someone who will not be around for several days. Looks like water and is harder to detect than the celophane on the lid. For a more instant effect, there is a substance availlable at most magic supply stores called anhydrous sodium poly-acrylate which holds up to 300 times its weight in water. Doesn't take much to turn a toilet solid or someone's drink, or...
Lydia