An attachment providing a second bubbling source for water pipes. This keeps the bong in a clean and pristine condition by catching any ash sucked through from the bowl, but an ash catcher should not be confused with a double bubble. A double bubble is a type of bong and an ash catcher is like a bubbler for the bong. Every ash catcher has an added bonus of further filtering carcinogens, and it's rumored adding hot water to the ash catcher will pull more carcinogens out than cold water; but ice cold water is wanted to cool and condense the smoke. By combining the two stages of different water temperatures the hit created from this will be a super smooth monster.
Picture of an ash catcher