Interviewer: You have a sheet of paper. As you can see there are two surfaces on it. You are locked in a room and have NOTHING with you. Convert this 2 surface sheet into a single surface.
Candidate: I will paste in on the floor, then there will be only one surface.
Interviewer: But you don’t have any glue to paste it.
Candidate: I will roll it in the form of a cylinder.
Interviewer: But then there are 3 surfaces (top, bottom and lateral).
Candidate: A sphere has a single surface. What if I make a sphere out of the sheet?
Interviewer: But how will you make a sphere when you don’t have anything with you?
After thinking for about 5 seconds. I picked up the sheet and made a paper ball out of it. Just like this: