Sex of flies
A woman enters her kitchen and finds her husband motionless with a fly-kill in his hand.
— What are you doing ? — she asks.
— I'm chasing the flies — he replies.
— Oh ! How many have you killed ? — she asks.
— 3 males, 2 females — he answers.
Intrigued, she asks:
— How can you differentiate between males and females?
— 3 were on a can of beer, 2 were on the phone.