I bought my son a toy hydrogen fuel cell car for Christmas.  It turns out that it is a great demonstration for my classes.  It is a great example of how energy is converted from one form to another. The car retails for around $70 online.  All that you need to add is some distilled water!  A description follows and I will add a link to some video.

The Hydrocar by Horizon Fuel Cell Technologies is an educational vehicle whose main fuel source is hydrogen. Water is poured into a tiny (polymer electrolyte membrane) fuel cell, which in turn is hooked up to a solar panel. The energy from the solar panel transforms the water into hydrogen and oxygen, which in turn powers the fuel cell of the tiny Hydrocar vehicle. The hydrogen is converted to electrical energy by the fuel cell.  An electric motor converts the energy into mechanical energy that moves the car using a gear system.

There is probably no end to the possibilities for demonstrations with this car.  It is Easy to set up and use… and it has a “cool factor” for your kids.  It also may represent the future in transportation.

Video:

http://www.youtube.com/watch?v=WEOrBntBlRw

Another description:

http://www.inhabitots.com/2008/10/28/smart-toy-hydrocar-for-kids/