Rubber grommet for brew lid

A rubber grommet for a fermenter lid, from Morgans. It seats into the hole in the lid and holds the airlock, giving you a snug fit so the ferment can breathe out without letting anything back in. Grommets perish, go hard and get lost in the bottom of the sink, so it is worth keeping a spare or two in the brewing box rather than discovering on brew day that yours has gone walkabout. Cheap insurance for anyone still fermenting under a standard plastic brew lid, which is most of us.