A man is washing the car with his son. The son asks, “Dad, can't you just use a sponge?”
Why this joke works
Let “With” Do the Work
“Washing the car with his son” normally means father and son are working together. The son's question changes “with” into its tool meaning, as though the father is using him instead of a sponge. Read the first sentence as an entirely ordinary family chore and avoid signaling anything strange. Give the son's question a tired, practical tone. The absurd picture should emerge only after the word “sponge” forces listeners to reinterpret the opening sentence.
A closer look
The Son Gets the Practical Last Word
The first sentence resembles a narrator establishing a wholesome shared chore. Giving the son the dialogue changes him from background helper to the only character who recognizes the bizarre alternative reading. His question is phrased as sensible advice, not alarm, so the imagined misuse stays cartoonishly impossible. Ending on “sponge” supplies a familiar object that instantly completes the visual.
Fact behind the joke
A Sponge Can Help Keep Car-Wash Water Contained
The EPA recommends washing a car on a lawn or other surface that can absorb water, using a bucket, and wringing sponges or rags into that bucket or a sink. Those steps reduce soapy runoff entering storm drains. In the real chore behind the joke, a sponge is useful not just for scrubbing but also for controlling where dirty wash water goes.