Suppose you take a line that's 50 feet long, and you position yourself at the center of it:
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Next, you look to either direction, and see two things that are each 20 feet away from you:
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This would lead you to conclude that the two things are 40 feet from each other. But the universe is curved - if you take either end of the line, and bring them together to form a circle with a diameter of 15 feet, then the two things that you are observing turn out to be only 10 feet from each other, not 40 feet.