Not a funny but a fun one...

When I was a full time primary classroom teacher I used to get my class to makes these mouse trap cars - it always provide great entertainment and provided a way for students to test hypotheses about the size of the drive lever, wheel base, wheel size and configuration etc.

The following video does a great job in detailing what you need to do - for those that work in metric the follow link may be of use - metric conversions.

Mousetrap Powered Vehicle! - A funny movie is a click away