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."