Kids are cruel. And geeks will be geeks. Given those two facts, the only thing that is better than lurking on a balcony above the tent that your sisters have set up directly beneath, waiting to tip a bucket of water on their heads as they emerge, is to automate the process, or at least make it remotely controllable. And thus we bring you the dump-o-matic.
Not much needs to be said that isn't obvious from the pictures. The solenoid once triggered the slide advance in a slide projector. The container is a tobacco tin.