chalk and cheese

Meanings

1. Said of things that are very different, though possibly superficially alike.

Etymology

Attested in variant form since 1393. The surface of some cheeses, particularly unaged ones, resembles the surface of chalk: white, crumbling, but in substance cheese is much softer than chalk. It is interesting to note that lime chalk and cottage cheese forms a glue-like material used in the restoration of ancient half-timbered houses, perhaps suggesting an alternative original meaning.