Airport screeners are now scanning holiday fruitcakes. Not even the scanners can tell what those little red things are.

Letterman’s joke riffs on the post‑9/11 airport-security era, when heightened screening made even mundane holiday items feel suspicious. The humor hinges on fruitcake’s notorious density and its candied fruit—especially the bright red bits (often cherries)—that look artificial and vaguely unidentifiable. By claiming that even imaging scanners can’t determine what the red pieces are, the line exaggerates both the opacity of security technology and the culinary mystery of fruitcake. The punchline also plays on a shared cultural stereotype: fruitcake as an enduring, dubious gift that seems less like food than an indestructible object.