Killing Flies

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, "Easy. 3 were on a beer can, 2 were on the phone."