Answer :
The answer is a I believe. Think of it this way: the bee _____ the bottle. Fill in a word there. If it makes sense, it's a preposition. I.e. The bee enters the bottle. The bee opens the bottle The bee is the bottle. There are a few acceptions, though.
Jonesie is correct, but the examples they provide with the bee are wrong. Words like enter and so forth are all verbs, not prepositions.