You'll need 2 film canisters (another relic), Alka-Seltzer, water and a pan with a line drawn down the center.
|I took the set-up picture with the transparent canisters, but later switched to the black**|
Fill both canisters about half full with water. You'll want to have the same amount in each canister.
Cap one of the canisters and lay it on its side so the cap is against the line in the pan.
For the next portion, you'll need to work quickly....
Add about 1/4 - 1/2 of an Alka-Seltzer tablet to the second canister and cap it. Then lay that canister so its cap is against the line in the pan. (The two caps should abut one another).
When the Alka-Seltzer creates enough gas to fill the canister, it will pop the top off. At the same time it will push the second canister. Equal force will be applied to each canister, but in opposite directions. After the explosion, the two canisters will end up in mirrored positions.
*If you want to do it on an overhead projector, draw a line down the center of a transparency using a permanent marker. And use a minimal amount of water.
**This is definitely a demonstration to play around with before you plan to do it in front of your students! I've done it successfully several times in the past without problems, yet when I went to photograph it, I ran into problem after problem. The first canisters I grabbed to use leaked so that enough pressure never built up to pop the top off. Then I used too large a piece of Alka-Seltzer and sent the canister flying out of the pan and off the table (fortunately it didn't go through the brand-new dining room window!). It's a great little activity, just give yourself a chance to practice it in advance!