Stu had no idea what mathematics he was doing, and had to let out a wheezy kind of laugh at what he hoped was a joke. They were joking together, this was great! Up until the point that Stu had to ask, "What, am I trapped here? Is this where they turn you green?" He did look concerned then, because he hadn't exactly been planning for an escape, he really did think he could just walk out of here any time. Billy did, and he wasn't even green yet.