How to Tell the Sex of a Fly

A woman walked into the kitchen to find her Husband stalking around with a fly swatter.

"What are you doing?" She asked.

"Hunting Flies" He responded. "Oh! Killing any?"

She asked."Yep, 3 males, 2 Females," he replied.

Intrigued, she asked. "How can you tell them apart?"

He responded, "3 were on a beer can, 2 were on the phone".