Buy a bottle of soft drink for each boy in your den. Open them and pour the contents into a large metal pitcher. Rinse out the bottles and let them dry.
Set up a "refueling" station in your kitchen, complete with goggles, plastic apron, rubber gloves, and other "scientific-looking" things. Make warning signs about radiation etc.
As the boys arrive, outfit them up and instruct them to "refuel" one soft drink bottle by pouring the mystery fuel through a funnel.