A guy who does home brew was telling me about them. I mentioned I have a good friend in Italy who is into all that kind of stuff so he gave me one to send to him. It's a plastic lid made by a company called Oztops. It replaces the lid on a conventional plastic juice bottle and functions as a kind of air lock. So the theory is you open the juice, add some yeast, brew it right in the plastic bottle. The Oztop lid releases pressure from inside the bottle with this little curtain thingy, ie. when the pressure builds up enough it forces the curtain apart and some gas is burped out.