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.