I appreciate the tip, and that explains, I think, the corks I've seen sort of crumble into little pieces when they were pulled from a bottle.

The one in there is one I scrounged somewhere. I have some others I bought from a craft store too, and compared them, but I don't see much difference except the craft store cork has voids in the sides, whereas the scrounged cork looks very smooth. Both have very smooth tops and bottoms, with no voids.