How to Tell the Sex of a Fly

This is kind of silly, but I couldn't resist. Besides, my son in law sent it to me. :)

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."