English: A drain "trap" uses gravity to keep some water at a bend in the pipe which prevents sewer gases from escaping into a bathroom or other human living space. Over time, the water will evaporate, and unless replaced with more water through normal everyday usage, it is possible that sewer gases will escape. Source: here entitled "Swapping out a bathroom sink" (handyman project) Note the words "public domain" note at the top of the article. Further questions, ask me (tom sulcer) on my Wikipedia talk page here.