Bubble, bubble, without toil or trouble using this classic double bubble airlock. A double bubbler offers advantages that other airlocks simply cant. It will show the difference between the inside air pressure and the outside air pressure (a great way to gauge fermenting activity), and it can also prevent "suck back" (a term used for when liquid in the airlock is sucked into the fermenter due to temperature changes). Not to mention watching the bubbles is a great way to pass the time while youre waiting for your beer to complete fermentation.