Milk

“If you were about to pour fresh milk from a container into a jug, but then you noticed that the milk that was already in the bottom of the jug was sour and lumpy, what would you do? You’d smash the jug, wouldn’t you?”

“No, why would I smash it?!”

“You’d hardly just pour the lovely fresh milk in on top of the sour stuff, would you?”

“I’d clean out the jug, and after a good wash then I’d pour the fresh milk into it.”

“Oh.”