Diffusers are the parts of a water pipe or bong that make the water bubble. The simplest way to explain how they work and the best comparison I could think of is a beverage in a cup with a straw. Remember when you were a kid and you would blow in the straw to make bubbles? Well that’s exactly what is happening when you hit your bong but in reverse. Instead of blowing through the straw (which would be the bowl and stem), you’re creating a seal on the entire cup (the mouth piece of the bong) and sucking (inhaling) to force air through the top of the straw (the bowl) to the bottom of the straw (the percolator). That’s right. You were playing with percolators before you even knew what they were!