What's the difference between a boat and a ship?
I've got a hypothetical question. I'm building this boat. It's getting bigger and bigger. At what point does it become a ship?
Well, we could get technical, I suppose. Among sailing vessels, the distinction between ships and boats is that a ship is a square-rigged craft with at least three masts, and a boat isn't. With regard to motorized craft, a ship is a large vessel intended for oceangoing or at least deep-water transport, and a boat is anything else.
But that's too much to remember. Try this: ships have to be big enough to carry boats, and boats have to be small enough to be carried by ships.
There are exceptions, of course. Many commercial fishing craft, for example, are sizable oceangoing vessels, yet they're almost invariably called boats. Similarly for submarines, built by General Dynamics' Electric Boat Division. The Great Lakes are pretty deep, and one sees certain large vessels on them that to all appearances are ships, but in fact said vessels are commonly called ore boats. However, these exceptions mar the classic purity of the answer above, so we'll pay them no mind.
Another question in the same vein, which we may as well get out of the way now that we've got the blood pumping, is: what's the difference between a hill and a mountain? We initially looked in the 1969 Random House unabridged and learned that a hill is "a natural elevation of the earth's surface, smaller than a mountain," while a mountain is "a natural elevation of the earth's surface rising more or less abruptly to a summit, and attaining an altitude greater than that of a hill." In other words, a hill is smaller than a mountain and a mountain is taller (and steeper) than a hill. Big freaking help.
Recognizing the inadequacy of the foregoing, the editors of the Random House dictionary took another stab at the problem in their second edition (1987). Hill stays the same, but a mountain is now a natural elevation, etc, "attaining an altitude greater than 2,000 feet."
This is a commendable attempt at precision, but it runs into trouble on the very next page, where we find a list of "Notable Mountain Peaks of the World." Mount Carmel, Israel, checks in at a paltry 1818 feet. Many of the so-called mountains in the Ozarks are similarly stunted.
Perhaps we should say that anything over 2,000 feet is automatically a mountain, but peaks under 2,000 feet may qualify if they (1) are steep, (2) have rocky sides, or (3) have the word "Mount" in their names. Of course, this doesn't help us out with the Black Hills of South Dakota, which reach a height of 4,000 feet above the surrounding country but don't qualify for an upgrade in nomenclature. But it's the best I can do for now.