My son asked me why I always carry a pencil behind my ear.

This joke gets its giggle from a classic bit of wordplay, specifically a pun that plays on the double meaning of the word "draw." When the setup mentions carrying a pencil behind the ear, our minds immediately go to the physical act of drawing or writing, picturing someone ready to sketch a masterpiece or jot down a note at a moment's notice. It sets up a very literal expectation of what a pencil is used for.

The punchline then cleverly pulls the rug out from under that expectation by shifting to the idiomatic phrase "to draw conclusions." This isn't about putting pencil to paper at all; it means to deduce, infer, or figure something out based on available information. The humor springs from this unexpected twist, taking the listener from the tangible act of drawing with a pencil to the abstract mental process of "drawing" a conclusion, all with the same word.

It's a charming example of how language can be playfully manipulated. The image of someone so dedicated to critical thinking that they keep a "conclusion-drawing" tool handy behind their ear adds an extra layer of lighthearted absurdity to the clever linguistic trick.