How Do Big Corks Fit into Tiny Wine Bottle Openings?

Before a cork is installed in a bottle, it is in cylindrical form with a diameter slightly less than the neck of the bottle. At that point, there’s no problem putting the cork inside, thanks to its ability to be easily compressed when an external force is applied.

Once in place, the cork expands and closes the opening of the bottle. This is why champagne corks appear mushroom-shaped when taken out; the curvature of the cork depends entirely on the shape of the bottle in which it is installed.