How to tell the sex of a fly

[Coyote]

A woman returned home one day to find her husband stalking around the house with a fly swatter.

"What are you doing?" she asked.

"Hunting flies." he responded.

"Oh.! Killing any?" she asked.

"Yep! Three males and two females." he replied.

Intrigued, she asked, "How can you tell their sex?"

He replied, "Three were on a beer can and two were on the phone."