Combine the water and raisins in a jar with a tight lid. Be sure to use non-oil coated raisins. Sterilize the jar. If it's too much trouble to boil it, rinsing it out with boiling water should do the trick.
The next step is to wait. The fermentation period varies depending on the temperature. It should take anywhere from 2 to 4 days. Gently shake the jar and open the lid once a day.
The raisins should become plump from absorbing the water, and they will fizz as they ferment. If they fizz loudly when the jar is opened, it's ready!
Story Behind this Recipe
I simplified a recipe for homemade yeast for beginners.
Don't forget to open the lid once a day. If it becomes too fermented, opening the lid can be dangerous as it may explode. If you want to be on the safe side, open the lid twice a day.