This is an amazing, fun youtube video of the band Ok Go and a Rube Goldberg machine. A great way to show kids the idea of a chain reaction and introduce basic aspects (and applications for) physics!
(I highly recommend you double-click on the video below to go watch it on youtube instead of here--you can bump up the resolution and make it full screen for maximum viewing pleasure!)
And I really like their "marching band" version of this song too--it is just so positive:
(this one would not let me embed, so you will just have to follow the link if you are interested :)