When I was in scouts, many times someone would slip a 10 pound rock into someone's pack as a practical joke. I suspect nothing has changed in the last 40 years. As an attention step I'd take a 10 pound rock out of my pack and say many carry a 10 pound rock that doesn't look like a rock. (Most likely it's extra stuff Mom threw in.)

I'd go through each system and explain how rocks can be taken out without costing any money. It might help for the scout master to bring his pack and use that as an example.

These kids are going to grow up, so I'd give suggestions for the future, too.