Why "Tongues of Fire"?

Happy Pentecost!

Today in the liturgy we hear the story of how the Spirit descended upon the disciples in the form of tongues of fire. 

Why does the Spirit appear in this way?

John has written a great post on the readings for today. However, since he didn't touch upon this aspect of the story, I thought I'd deal with it in a separate post.

In short, I think the "tongues of fire" imagery is