Elementary, Dear Watson

Quite apart from the new Sherlock Holmes, which is tres fun, this is logical problem solving second grader-style:

Son: You know, only one percent of the water on the planet is drinkable. We'll really be in trouble if we don't look after it.
Daughter: We can just drink milk then.
Son: What will the cows drink? They need water too. And for grass. One day, there'll be no cows.
Daughter: So then, we'll just have to learn to milk a walrus.