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.