This is what a standard brewer's airlock looks like. It is used in brewing to produce a closed environment for fermentation. It can be made of plastic or glass. The plastic ones cost about a dollar and the glass ones around ten. The glass ones usually on have one ripple on each side, while the plastic one i drew below has three. You fill it with a small amount of water and as the yeast ferments, the CO2 it produces can bubble up through the airlock and escape. This way CO2 can leave the bottle but nothing can get back in. It keeps the yeast's environment sterile.
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