you could start with a timer: take a container (possibly a big pop bottle) and put a small hole in the bottom. then place a second container under the first container so that it catches the water leaking out of the first container. cut a hole in the side of the second container and put in a hose or pipe for a siphon tube. (one end on the inside of the bottle near the bottom, and the other end outside the bottle near the bottom). silicon rubber the hole around where the hose goes through the side of the bottle so it is water tight. when the second bottle fills with water to the level of where the siphon tube goes through the side of the bottle, the water will rapidly siphon out of the second bottle.
ust for giggles have this water fill another bottle set up the same way but so that it holds slightly less water (to make sure there is enough water in the second bottle to make the third one start to siphon).
Next have that water fill another bottle with water and as it fills and gets heavy enough it sets off something like a mouse trap.
instead of the mouse trap you could see if you could use the guts (motor or something) out of an ole vcr hooked to some batteries.
or control a switch that runs a drill that tightens a screw or bolt in two boards that smash the tooth paste tube.
or use the bottle to set off a mouse trap that trips a rat trap that sets off a coyote trap that smashes the tooth paste.
do you want just a little tooth paste, or do you want the tooth paste to hit the tooth brush sitting across the room!!!!
Better put those bigger traps inside of a wire cage or something so some fool won't get their fingers broke!!!!
Have fun!! and Good Luck