Think of the mighty wind the Spirit of God blew across the dark waters on the first day, when Genesis says God created light. Just as the breath of God breathed new life into the creation in the beginning, the same thing was happening to the church on the day of Pentecost. What was first heard was then seen. Tongues of fire appeared among them and a tongue rested on each one of them. That's about the closest analogy I've ever heard applied to this text.
Fire is too hot to handle, it is energizing, it spreads quickly under the right circumstances.
And in this case it also represents judgment, judgment against those who fail to receive the gospel message. Their elocution was so good that they began to draw a crowd. You see, Jerusalem was a very diverse place with devout Jews from all over the world living there. The crowd was absolutely bewildered as they listened, and one wonders if those speaking the languages didn't feel the same. Perhaps they didn't even understand themselves what they were saying.