What I love about this method is that it includes a way to take any cork and make it fit a bottle using the magic of microwaves!
This project was originally inspired by the howtoons about bottle rockets. My friend Will Macfarlane and I played with it a lot but found it tough to do with kids ages 6 - 8: the inner tube stopper would come unwrapped and kids had trouble putting it back together, let alone making it on their own. We got into microwaving everything in sight one week, including everything in Star's microwaving instructable, and realized that corks expand when microwaved.
This led to a new way to make bottle rockets! We've used this at Camp Kaleidoscope since '07.
Step 1: Preparing the cork.
We think this works because water in the air inside the cork gets hot and expands, causing the cork to expand. The whole thing cools really quickly -- I've never had to do anything more than put it in the bottle, leave it for a minute, and then take it out.
For soda bottle size, I like 20 oz. or 2 Liters: they have the same opening size, so an opening made for either one will work on both.
We've found that microwaving for 25 seconds worked well, and a lot more would burst our corks. The exact time will depend on the power of your microwave.
You don't always have to microwave the cork -- sometimes the cork will fit naturally into the bottle. What's a nice about this method is that it's not a problem if it doesn't. Rubber stoppers from chemical suppliers will work too, but this is a scarcer item. I think size #4 rubber stopper fits 20 oz/2-L bottles.