Characters
Light the lamp, not the rat. Light the lamp! Not the rat!
— Rizzo the Rat ‧ The Muppet Christmas Carol
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Rizzo is shouting an urgent instruction, and the joke is the rat he doesn't want set alight. The unlit lamp is the thing that's meant to be lit. It turns a frantic cry into a plain, funny correction.
Context reviewed