So what makes a Rolex Oyster an Oyster? The winding crown acts like a screw-on cap, or micro jar-lid, and has watertight gaskets in it. The best way to think of it is to imagine a submarine hatch that is threaded and has to be twisted or spun open or closed.
Hans came up with the idea of associating the fantasy of mermaids with the waterproof Rolex Oyster as seen in the 1926 French Rolex advertisement seen below.
1926 French Rolex Mermaid Oyster Poster