The Hack
Adult fleas are attracted to heat sources. Incandescent light bulbs emit more energy as heat than they do as visible light, so a small bulb can be used to lure fleas to get trapped in the soapy water or a sticky pad.
This won't catch every flea in a room, but it can be used a test to measure flea activity.
This won't catch every flea in a room, but it can be used a test to measure flea activity.
Limitations
Only adult fleas are mobile enough to jump towards a heat source effectively, but adult fleas prefer to stay permanently on their host. This means that only fleas which have somehow become dislodged are vulnerable to this hack.