When we were taught this in class, I imagined it on the khlaw, using rocks for points. You calculate the distances between all three points. If the sum of two smaller distances equals the largest one exactly, they lie in a straight line. But, my friend, here the graph paper is scarce and the textbook examples are too neat—in our hills, even a small rounding error in your arithmetic can make three stones seem not collinear! My own proof often waits for a clear evening when I can check it against the bamboo pole on our roof, laid straight against the sky.
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