I can't cook. I use a smoke alarm as a timer.

This is a self-deprecating one-liner that turns domestic incompetence into a punchline. By claiming to use a smoke alarm as a “timer,” the speaker implies meals are routinely cooked to the point of burning, and that the only reliable signal to stop cooking is an emergency warning. The humor comes from the mismatch between a kitchen tool meant for precision (a timer) and a device meant for danger (a smoke alarm), suggesting chaos where competence is expected. As a quip, it also plays on modern anxieties about everyday skills and the social pressure to be “good in the kitchen,” defusing that pressure through exaggeration.