Suspended steel filaments make three-dimensional magnetic fields visible.
I came up with this one for the Electronics exhibit at Museum of Science, but it ended up as a build-it device for science teachers.
I also discovered that baby oil slowly pushes its way through glued plexiglas joints. If you build an 8" cube of thick plexiglas and fill it with baby oil, a few months later you notice that it's half empty! And the other half is soaked into all the books on the shelf below! AND IT'S A SHELF IN A COLLEAGUE'S OFFICE!!! :)