The Submarine Science Project
Several years ago, my brother had to do the school science project where you're required to "make a submarine that submerges and re-surfaces". This is one of those things where, if one of my kids were to get that assignment, I would spend hundreds of dollars "helping" them make the most perfect toy submarine possible, with solenoid-controlled Main Ballast Tank vent valves and those little CO2 cartridges to blow the water out; it'd all be radio-controlled, of course. Back then, though, I was working long distance, and I think the project was due the next day. So, if you find yourself in this situation, here's what you need to do:
1) Get some raisins (not the kind from a "Raisin Bran"-type cereal, though; if that's all you got, you'll have to wash off all the sugar coating.)
2) Fill a glass with vinegar
3) Add a tablespoon of baking soda
4) Put the raisin in the glass
The reaction of the baking soda and vinegar releases a gas (probably carbon dioxide -- it's been 17 years since I got my Chemistry B.S.) which collects as bubbles in the crevices of the raisin. When enough bubbles collect, the raisin rises to the top of the glass as the bubbles displace enough water, whereupon the bubbles burst due to the lower pressure. Freed of the gas bubbles, the raisin sinks, and the process repeats. My brother got a 100% on the project, as I recall. (Mom, you reading this? Did I remember the story right?)
Actually, as I did some research after I wrote this, it turns out that you can do the same thing just by putting raisins in a clear carbonated beverage.