How to find one (two): The temperature at the spot (B) on the other side of the world from you (A) is either the same (Finished) or different. If different, head towards B, (any direction is fine!) and at some point before you get there and the A vs B temperatures are swapped, the two will be the same. Since the direction you go doesn't matter, evidently there's a path "around the world" where this true, not just a point! (Ok, just a point if they're initially the same.)