We recently discovered this premium breathable silicone carboy hood and were immediately impressed with its robust design and features. To start with, the hood is designed to fit over the top of your carboy so you never have to worry about an airlock/stopper popping off of your carboy Where the hood really shines is in its two-piece design. When set to the open position, the bung acts as an airlock and allows the release of CO2 while maintaining a seal against oxygen coming in.? When fermentation is over and you no longer want the bung to "breathe" out, you just push the center cap into the bung to create a firm seal and the FE496 becomes a solid stopper! The FE496 fits all of our glass carboys and our 5 gallon and 6 gallon "Better Bottles".