My son asked me how birthdays work.

This joke cleverly uses wordplay and a touch of irony to get its laugh. The humor hinges on the double meaning of "offensive." While we typically think of something offensive as being rude or insulting, here it's applied to the increasing numbers of one's age. The idea that a number itself could be "offensive" to someone who is getting older creates an amusing incongruity, a common source of humor where expectations are delightfully subverted.

Birthdays are celebrated universally, often seen as joyous occasions marking another year of life and personal growth. However, there's also a widely relatable sentiment, particularly in Western cultures, that as we get older, the ever-higher numbers can feel less like a celebration and more like a humorous, if sometimes slightly unwelcome, reminder of the passage of time. This joke taps into that shared experience, playfully acknowledging the mixed feelings many adults have about their advancing years, especially when those milestone numbers start rolling in. Humor, especially about aging, is often used as a coping mechanism.