The Solo Stone: Where is Jon's Other Head?

Jon looked at himself in his mirror and saw that while he was dressed and looked well-showered, there was something definitely wrong, because normally he had two heads! *Everyone* had two heads. It was so freakish to see himself with only one. How could he let people see him like this? They'd call him a freak right on the spot. *Maybe I could wish to have my other head back,* Jon thought to himself. But why did he lose his other head in the first place?