Determining Fly Sex

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?'

Simple,he responded,

3 were on a beer can, and 2 were on the phone.