When teaching English to non-native speakers, it's the little words that are the most difficult. In particular, prepositions can be really pesky: They don't translate exactly, if at all, from one language to another. I still don't know how to explain why someone lives in Queens but on Long Island.
Likewise, why do postal carriers deliver on bicycles, but whatever they bring arrives by bicycle?
Well...at least we know they're not delivering in a bicycle--though the letters, cards, bills and such might be in a bike when they arrive. At least, in a bike like this:
