Being for the Benefit of Mr. Ross

Regarding Timeslides: What exactly does Rimmer mean when he says, “He always used to come bottom in geography. He thought a glacier was a bloke who fixed windows.” Now, excuse me for being thicker than a whale omelette, but I’ve never been able to make any sense of this gag, no matter how many times I’ve heard it. I find it somewhat ironic that it’s the gag about Thickie Holden that I can’t understand, but that’s how it is. Can anyone explain this joke? Is it something so simple that I should clearly have been able to understand it? Or, is it something that I wouldn’t be able to understand/know, not being born and raised in Ye Olde England?

Are there any jokes that you weren’t able to understand until someone explained it to you? Surely some of you have suffered from this debilitating condition? Surely I’m not the only one? Oh, I am? All right, then.