These are cool to show whirlpools (or fake tornados) to kids. The write-up is pretty good on how to make one, but I like the 2nd version the author came up with better.
http://www.instructables.com/id/Computer-Fan-Magnetic-Stirrer/
v2
http://www.instructables.com/id/Make-your-own-Magnetic-Stirrer-2S/