Here is a video of "the head butt":
You want to know WHY he did that? WHAT could have possibly set him off like that?
Now watch this video all about Marco Materazzi. You might want to turn the volume down -- the song in the background is pretty obnoxious.
Zidane apologized to all children everywhere for his actions, but said that he could not live with himself if he didn't do what he did. I can only imagine what Materazzi must have said about Zidane's family.
I know a lot of this "dirty playing"goes on in soccer, but I think Materazzi takes it to a new level. I probably would have done what Zidane did.