From publisher blurb:

Players who find themselves trapped within this chamber must search the pools that lie within for the four keys needed to unlock its door. The only problem is, some of the pools aren't so cooperative?

Sample:

Invisible Pool
This seemingly empty pool is filled with a failed invisibility potion. The liquid itself has turned invisible. Anything touching the pool will feel wet but look dry and, of course, still be visible. A copper key sits at the bottom of this pool. Anyone reaching for the key will be in for quite a surprise as soon as they come into contact with the invisible liquid. A potion bottle filled with this liquid will appear to be empty, even though it will make a sloshing noise when shook and gurgle when poured.

A line of unique and challenging chambers, designed to be dropped into an already existing adventure; Add-A-Rooms are quick and easy ways to add a little something extra to your game. Each Add-A-Room features elements that can be re-used in other adventures, such as magic pools, fake treasures, or ingenious traps.

The Add-A-Room line is part of System 20, a design your own supplement system created by Creation's Edge Games.