grammar - What part of speech is "on" in "on fire"?

A while ago, there was an answer on Jeopardy! along the following lines:

In the sentence he was on fire, the word on is this part of speech.

The judges ruled that it was a preposition. But I think that only works if the guy was physically located on top of a fire. After discussing it with someone, I think it's the first half of a compound adjective. There's a convenient parallel to the single word "afire" there.

So, who was right? And why?

Answer

The preposition "on" has a number different meanings, one of which is to "physically be on top of something". The preposition in the sense of "on fire" is one of them (from dictionary.com):

13 . in a state or condition of; in the process of: on strike; The house is on fire!