Two Ropes on a Deserted Island

Context: This riddle was told to an audience of one in a fraternity dining hall.

Background: The informant learned this riddle during a long car ride, when he was trying to pass the time.

Q: You’re stranded on a deserted island with nothing but two ropes, and a lighter. There is an unlimited amount of gas in the lighter. The ropes vary in thickness and material throughout the entire rope. The only thing you know for certain about the ropes is that they each take exactly one hour to burn. How do you measure 45 minutes?
A: Light the first rope from both ends at the same time, and light one end of the second rope. When the first rope has burned up completely, 30 minutes have passed, and there are still 30 minutes of burning time left on the second rope. If you light the other end of the second rope, it will burn for 15 more minutes, 45 minutes total.

This riddle requires abstract thinking, and has a very elegant solution.