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