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.