I have always wondered, what is under Edd's hat?

Ever since i was little and started watching the show, the question that has more then likely filled the minds of anyone who has seen, what exactly is under Double D's hat? I have heard many rumors from friends but yielding no results. I even heard that its mystery is revealed in the final episode (Big Picture Show) but had disappointing failure, granted the show was still amazing. So I decided to ask what exactly does the top Edd's skull look like?