This is one of those impress your kids at the dinner table kind of activities. Ask them to balance 2 forks and a quarter on the edge of a cup and only the quarter can touch. It's pretty funny to watch them try and come up with something that works. Here's how to do it.
Start with a cup, 2 metal forks, and a quarter. If it's a paper cup or some other light material make sure it has some liquid to weigh it down. I think the boys put water in to see it splash out when the quarter or forks fell in.
You put the quarter in the middle of the fork teeth with the forks facing the same direction like this
Then you balance the forks on the quarter which is on the edge of the cup. You adjust the positioning of your forks until they balance. Our forks are pretty curvy so they needed some adjusting.