[yes, he had prepared himself for this one. this time, there's only a small pause to adjust to the idea of someone having no idea what he's talking about before he continues.]

Because there are two types of water--salt water and fresh water. Fresh water comes from lakes and rivers and falls from the sky, and the salt water is out in the oceans. Imagine water in every direction for uncountable miles, but deadly to drink.

[simple as that. what more is there to know? and just as offhandedly, he nods to a beautiful little establishment coming up on their left. a quaint little cafe, but what sets it apart from all the others is the amount of flowers in the windowboxes and little garden spaces out front, and teeming over the edges of the high walls of a courtyard attached to the side of it. the place has a definite theme and the owners are obviously milking it for all its worth, cultivating a veritable museum of flowers to draw in passersby. the outside of the building is beautiful, but the half-hidden garden beyond the walls is the real prize, teasing potential customers with the plants perched on and overflowing over the top of the fence.]

...That had been your other option, by the way.