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."