Thumb
The first digit of the human hand. It has only two phalanges and can be moved to oppose the other four fingers, making grasping and pinching possible.
Close one eye, extend your thumb to align with a distant object, then switch eyes. The object appears to shift. Multiply that apparent shift by 10 to get the actual distance. This works because the ratio between your eyes and your arm length is roughly 1:10.
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