On a recent visit to Bodegas Gomez Cruzado we discovered these funky looking things aptly known as eggs.
Apparently this is the winemaking equivalent of a big wooden spoon used to stir the grape juice and yeast during the fermentation. You need to stir the mix to ensure that all the juice is equally exposed to the yeast.
A bit of science for you: basically the liquid in the narrower part of the egg is naturally warmer than the liquid in the wider part of the egg due to there being less volume. The molecules in the warmer liquid move more than the cooler liquid causing it to naturally flow slowly round the egg, thereby exposing all the liquid to the yeast.
I assume the big steel vats used in modern bulk wine making have some sort of mechanical stirring system. I’ve no idea which is better but the eggs are definitely cuter. I like them.