I've been tutoring science for some high school kids for several months now, and this week we finally got our hands dirty and put together a copper sulfate and zinc battery for a school project, which was pretty neat. It went over really well with the family. So well in fact that they want me to start doing a weekly experiment/demo with the kids. Now my brief Google research turned up a few interesting demos, but a lot of them were just the usual little kids fare, like cornstarch, baking soda and vinegar, borax gak, etc. I'm looking for some safe but slightly more sophisticated demos more appropriate for high school kids. And I figured people here at PF had to have some favorites, so let's hear 'em! Any ideas are appreciated, from physics to chemistry to biology to whatever. Also, cheaper is better as always.