Anyone know a short joke with the punchline: 'penis'?

I need help thinking of a joke that results in the simple punchline: 'penis'. Its difficult for me to explain what this is for. I'm kinda making a visual joke within a text message where I write "8====D" but I need something funny to lead into it.
I know this sounds really dumb, but there is more to it than this (involving screen capture of the text and photoshop), but any help would be greatly appreciated.

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