How to tell the Males Flies from the Females

A woman walked into the kitchen to find her husband with a fly swatter.

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“What are you doing?” she asked.

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“Hunting Flies”, he responded.

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“Oh! Killing any?” she asked.

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“Yep, 3 males, 2 females”, he replied.

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Intrigued, she asked, “How can you tell them apart?”

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He responded, “3 were on a beer can, 2 were on the phone.”