Why do Milk and Juice Cartons always have Liquid Left Over after you pour them out?

It happens when you’re “emptying” all sorts of containers, including cocktail shakers and wine bottles. I hadn’t given it much thought, but you have inspired me to figure out what really is going on.

What’s undoubtedly happening is that as you “empty” the container, some of the liquid encounters microscopic rough spots or non-wettable spots on the container’s inside surface.

These spots hold back small drops of the liquid, which remain stuck there as long as the container continues to be inverted. But when you return it to its upright position, the small drops can slide back down, because the path had been smooth up until they encountered those snags.

So the drops do slide back down, joining their brethren at the bottom to form a pool. The pool is now heavier than any individual drop, so when you invert the container again the pool can steamroller right down past the rough spots.

I hope you are happier for understanding that. I know I am.