You find yourself trapped atop an $800$ foot tall building. The surrounding land is completely flat, plus there are no other structures nearby. You need to get to the bottom, uninjured, and can only safely fall about $5$ feet.
You look down the four walls; they are all completely smooth and featureless, except that one of the walls has a small ledge $400$ feet above the ground. Furthermore, there are two hooks, one on this ledge, and one directly above it on the edge of the roof. The only tools you have are $600$ feet of rope, and a knife. How do you get to the bottom?
Source: I got it from this puzzle collection, but I think it has been used in interviews.