In/on

[Xinfu]

What's the difference between in and on?

-...he had been with his uncle in a lighthouse in a storm.
-But how long do they leave men on a Lighthouse?

[poli]

in the lighthouse means inside the lighthouse.
on the lighthouse means on the walls or on top of the lighshouse. Example: Seagulls perch on the lighthouse all the time. or The workmen are on the lighthouse now repairing the storm damage.

[Glen]

That's a confusing bit alright, Xinfu, and I'm like poli in automatically thinking of in as meaning inside something.

And at least when it comes to public transportation I like to think of on as meaning on board, so that one can be on (on board) the bus/plane/train/ship. But as for private transport one is said to be in (inside) the car/taxi.

Hope it helps.

Oh, it just occurred to me that one is said to be on a bicycle/motorcycle, so there goes my theory. English speakers, help us out here!

[wrholt]

If we ride it to be transported someplace, and we are supported outside of it, we are on it: bicycles, motorcycles, horses, camels, elephants, etc.

If it flies, in general we are on it: airplanes, helicopters, blimps, etc.

If it floats, if we can easily walk around aboard the vessel without falling into the water, we we are on it: yachts, cruise ships, battle ships, rafts, etc. However, if it's relatively small and standing/walking around has a high risk of capsizing the vessel, then we tend to say that we are in it: canoes, rowboats, dinghies.

If it travels on land, then if it's more-or-less possible to walk around inside of it, we are on it: trains and buses. If we can't stand up and move around inside of it, we are in it: cars/automobiles, trucks, semi-cabs.

There's a few types of conveyances that I'm not sure about, such as recreational vehicles/RVs (I prefer "in") and the trailers pulled by semi cabs (in this case I'd probably say "in").

As for buildings/structures that have insides and outsides, if we are inside, then we are "in" the structure; if we are outside and supported by it, we are "on" the structure. However, when a structure has more than one level and we want to describe location within the structure, it depends on the name of the particular level: we generally are "in" a room, cellar, basement, attic, but we are "on" a floor/level.

[Glen]

That's the best explanation yet; I'll be sure to use it myself