The lettering on a cookie seemed to survive Cookie Monster's teeth and digestive fluids, in one of his non-Prairie "Letter of the Day" segments. He'd eaten one with an X on it, then admitted "Me feel tremendous guilt!" over destroying the thing. When a nurse came in with an X-ray machine, her picture of Cookie Monster's insides revealed an intact red X floating around where his stomach should be. Granted, that's as close as Sesame Street "canon" gets to having a whole cookie survive the process...and granted, rubber ducks (even in a fictional world) aren't made of the same stuff as that letter-frosting. Still, Ernie's never lost his duckie to the monster's appetite, either on the show or in a book...which is a Good Thing, since whatever Cookie eats is typically gone forever. (It'd be hard to replace, too; I've read that the style of duck Ernie's owned since Season 1 isn't being made anymore.)