I do the same by magnetising a sewing needle, floating it on a bit of tissue, watching the needle point N, watching the tissue sink leaving the needle supported by surface tension, making it do tricks by holding the magnet secretly in my hand not-very-near it and doing "magic movements" with a finger of the other hand, the putting a drop of detergent on the water and -- as you say -- watching what happens.

Another favourite trick, with older pupils, is to make a Cartesian diver and appear to make it move with either my nose, or by getting it to follow a finger. I then give the kids the wherewithal to make a diver, and get tehm to figure out what's going on. Bingo -- Archimedes' Principle, buoyancy, etc. learned and I haven't had to teach a thing!

--

Stephen