Fill balloons with helium and tie a string to each one, make sure the string is is long enough so if it floats to the ceiling it can be retrieved. (This seems like a no-brainer, but I learned this the hard way working in a large cafeteria with vaulted ceilings) I bought the helium tank on amazon, it was way easier than trying to move helium filled balloons from the store to my program in my Impala. Fill enough balloons so that each team or individual has one, plus a few in case of incidental popping.
1. Ask kids what they know about helium. They will likely say something like. "It makes balloons float" or, "It makes your voice sound funny." and while everyone a good laugh when they suddenly sound like a member of munchkin town that is not the objective of this activity.
2. Next ask the kids what they know about gravity. Discuss how gravity is the pull that keeps things on the ground, and without it everything would float around willy-nilly.
3. Ask the kids why they think helium defies gravity. These answers can be interesting. The point is to get them thinking.
Ask the kids to use the materials provided and counter effect the helium by adding just enough weight that the balloon "Flinks" (Neither floats nor sinks but maintains a hoovers without hitting the ceiling or the floor) . Encourage them to change their design until the balloon flinks for 10 seconds or more.
Last time I ran this activity one kiddo was able to make their balloon flink for over four minute. But to be fair we were working in a room with a vaulted ceiling. :)