The tumbler-and-soap technique worked like this: A woman would look for a fly hanging upside down. She would then bring her tumbler of suds directly under the fly very slowly, taking advantage of the fact that an upside-down fly, when approached by danger, will drop straight down two inches or more, in a free fall, before using his wings. Ideally, the fly would not sense danger until it was directly below him, and he would obligingly drop into the suds to be caught, to work his way down into the bubbles, to drown.
Of this technique Eliot often said: “Nobody believes it until she tries it. Once she finds out it works, she never wants to quit.”
I don’t think I’ve tried it. We just don’t have many house flies around here. Anybody?
source: God Bless You, Mr. Rosewater by Kurt Vonnegut, Jr