My brother was adopted. Somebody left him on the back doorstep when he was a baby. We found him when he was 16. We didn't use that door.

Delivered as a stand-up one-liner, the joke hinges on a deliberate misreading of the sentimental trope of an abandoned baby “left on the doorstep.” The first sentence invites a conventional, sympathetic narrative about adoption; the next two lines abruptly literalize the image and stretch it to absurdity—finding the “baby” at sixteen—then cap it with a deadpan practical detail (“We didn’t use that door”). The humor comes from incongruity and escalation, but it also lightly satirizes how family stories can be packaged into neat myths: the speaker treats a dramatic origin story as if it were merely a household logistics problem.