How to Tell the Sex of a Fly

From an email:

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."

Too cute.