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Put a coin in a plastic tray. Consider both as a two-dimensional object. The coin can move within the tray but cannot escape from it. Suppose you lift the coin from the tray, it escapes from the boundaries of the tray. That is, you have moved the coin into the third dimension, that is why it escaped. Now put a marble in a box. Both are 3D objects. You cannot move the marble outside the box without damaging the walls of the box. But, if you move the ma...
