The Hack
Fill a dish or plate with water mixed with dish soap. Place it near where fleas are active or suspected. Any fleas that jump into the water will be unable to float because the soap breaks the surface tension and they will sink to the bottom and die.
Use
This hack is helpful for verifying that fleas are present but it isn't intended to eliminate fleas entirely, as it does nothing to draw fleas out of their hiding places or hosts.
Limitations
There is next to nothing to attract the fleas to the dish, so this relies on fleas randomly jumping into the water. Unless there is a heavy flea infestation, this may not even detect any fleas at all. An upgraded version of this trick is the Light Trap, where a light is placed above a Dish Soap Trap or a Sticky Trap as a heat source to attract fleas.
LIGHT TRAP HACK