(And another one...)

There was a guy in his kitchen swatting flies. They were buzzing all over and driving his wife crazy. After a few minutes she comes into the kitchen, and whines "You haven't even killed one fly yet. What is wrong with you??"
"I have killed 5 flies, thankyou very much, three of which were males and two were females."
"And Just HOW can you the sex on those tiny critters especially after they have been squished??"
"Ah Easy!", the husband replies. "Circumstancial evidence--3 were on the beer can and 2 were on the phone.