Alka seltzer tablet, within useby date
35mm plastic film canister
Do not tell the students what will happen. Get students to do a rehearsal with the film canister, but no tablet or water. All together, pretend to put the tablet in the canister, half fill with water, put upside down on the bench.
Then do it all in unison, and watch the canisters fly sky-wards. Get them to wipe up.