I was under the impression that all perler beads were round, how did the inside fire beads wind up so square?

When you iron the beads, the plastic melts down a bit and expands out, which forces all the once empty spaces to fill with the melting plastic. That's also how they all melt together and connect on the side that you iron. The more you iron, the flatter it becomes and you can completely seal any holes in between the beads if you iron long enough. Hope that makes sense. ^^() Here's a picture that might help illustrate my explanation better - http://www.furaffinity.net/view/16124487/