What causes the seasons?

If you are reading Game of Thrones right now, as I am, you know, like the Starks, that "Winter is coming." [Please will someone start reading Game of Thrones - I want to talk to people about it!].  Apropos of the changing of the seasons, Kevin Drum links to dialogue from a Calvin and Hobbes cartoon where it is explained that the seasons are caused by the Earth's orbit and its distance from the sun...Wrong! However, this is a VERY frequent mistake.  I once read a poll where over HALF of polled Harvard students actually answered the question "What causes the seasons?" incorrectly.  Unfortunately, I cannot find a link, but it was answered incorrectly by something like 60% of the respondents. So, without looking it up in wikipedia, who can answer the question correctly? BTW, I do know the answer.