Airlock

From Macdiyvers

For most diy booze projects you're going to need an airlock for your containers, to let the CO_2 out so your bottle doesn't explode, without letting any oxygen in. The easiest thing to do to relieve pressure is to just use a balloon, but people often complain of it tasting bad afterwards, so you probably don't want to do that.

Instead, simply connect a hose with an air-tight seal over the mouth of the container, and submerge the other end into a large bucket of water. Make sure the end submerged in water stays submerged the entire time. The bucket of water acts as a makeshift one-way valve, allowing any CO_2 to escape in one direction (without the backpressure a balloon would have) while stopping any fresh air flowing back along the hose because it is submerged in the water.