Claustrum Fermentationis
The fermentation lock, under its proper name — so carbon dioxide may leave the bottle without inviting the room back in.
$6.75
Brewers call this a fermentation lock. The catalog prefers claustrum fermentationis: claustrum, a lock or bar (and, in another mood, a cloister); fermentationis, of the working of the bottle. There is no older Latin for the object. The device is modern. The grammar is not.
Once yeast is added, the bottle will produce carbon dioxide. Carbon dioxide would like to leave. Without a claustrum it will leave anyway, usually through the cap, the carpet, or a conversation you did not schedule.
This is the appropriate lock. It fits the bottle we sell. It may fit other bottles. We bless the one we ship.
Do not put the claustrum on an unopened bottle of juice and wait for a miracle. The miracle, such as it is, requires the yeast.