[prewett]

Flies feel the rush of air from our oncoming hand and get out of the way, just like we look up from our book (phone, these days) when we feel air from the oncoming subway train we are waiting for.

The secret to killing flies is that they have to fly up first before they can fly away, so you (slowly) put your hands in front and behind them and "clap" them. They feel the air and lift off, right into the center of your oncoming hands.