in my chemistry class i took a decent sized rubber cork that has a whole going through it. so i shoved a nice socket(from my wrench set) halfway down the hole so the top of the socket is visible. Well, i use it as a G cap(gravity bong cap) it fits a 20 oz or a 2 litre bottle perfect and makes an airtight seal. basically it makes it so u dont have to unscrew the cap.