Mousetrap Cars – Any advice?

I have to get 7 cub scouts started tonight on creating a mousetrap car. This is in addition to the pinewood derby car they are already working on, and some are working on LEGO robotics creations as well.

I’ve Googled it, bought some supplies (but still don’t have the mousetraps), but lack any real practical experience in doing this myself.

Anyone done this before and have some words of wisdom/warning?