For people who don't drink a lot, 5 gallons of failed beer is just too much to drink. Additionally, when experimenting with recipes, it's nice to be able to test several batches at once to get the mix just right, testing each iteration side by side.
For these cases, it's good to have a quick-and-easy fermenter that's small enough to do quick runs.
Step 1: Collect Components
You probably have all of this in your house. I did, which is why these were the materials used.
1 - The base of the fermenter is a bottle of soda, any size. Here we use a 2 liter bottle.
2 - Airlock. This is a cheapass one from the brewstore.
3 - Plumber's Putty
4 - Hole making tools
Step 2: Prepare Lid for Airlock
Drill a hole the same diameter as the airlock input. The closer this hole is to the airlock the better, the friction will hold up the airlock and make the seal a little better.
The plastic melts if the drill bit is too fast, so go slow, and you will still probably need to trim off some plastic with a knife.
Step 3: Prepare Your Wash and Cap It
Brew up whatever it is your fermenting and add it as you would in a larger batch, then cap it off with your new cap.
Step 4: Insert Airlock and Seal
Put the airlock in the hole in the lid and place a snake of the plumber's putty around it, make sure the seal is airtight.
This cheapo airlock is molded all wrong for this, so you'll have to get a really good seal on the putty. The more expensive 3-piece airlocks have flat bottoms and can be pushed down to make a really good seal.
Step 5: Test!
Using this method you can make as many batches as you want to simultaneously for really cheap. It allows you to really fine tune recipes and gives good control over experimental batches. As a final note, I'd suggest using the same starter to pitch each batch, ensuring consistency in your tests.