10/05/2026
Have you ever found water in your washing machineâs detergent dispenser after your laundry was done and wondered why?
Itâs because many dispenser compartments (most commonlyâbut not limited toâthe fabric softener compartment) are Pythagorean siphons, also known as a Pythagorean cup, Tantalus cup, or âgreedy cupâ!
The animation in this post was created entirely by me and shows a cross-section of how laundry products empty out of them. đ
This type of siphon is an ancient design, with the oldest surviving example dated to 4th century A.D. and the first written mention as early as 1st century A.D. When the level of liquid in the âcupâ is below the top of the inner tube, the liquid will remain in the compartment. However, when liquid fills past that point, gravity begins pulling it down into the inner tube, causing the liquid to fill the space between the inner and outer tubes completely. This creates an airtight sealâand a vacuum.
That vacuum continues to pull the rest of the liquid out of the cup through the inner tube until the liquid falls lower than the rim of the outer tube, at which point the siphon âbreaksâ and stops the vacuum. Once the liquid level falls below the outer tube, itâs kind of like trying to pull liquid out of a container using a straw thatâs too short to reach whatâs at the very bottom.
This design is what allows laundry products to remain safely inside of their compartments in the dispenser until the correct point in the cycle when the water valve for that compartment begins to fill, beginning the siphoning process. Top-load washing machine fabric softener dispensers (and American-style flush toilets) operate using this same method!
This design is also the reason why itâs important to dry out your dispenser drawer when your washing machine is not in use. If you donât, moisture will sit inside that compartment and form mold, which can then make its way into the rest of the washing machine.
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