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In case of an arm: first have the upper arm ‘look at’ the target. The hand will not be at the right position yet. Then you rotate the upper arm by an angle determined by the magic ‘cosine rule’. Finally let the forearm point to the hand using the now familiar ‘look at’ function again. The tricky part is to determine the axis for the rotation. This actually happens to be the vector pointing upward from your hand, opposite of the palm of your hand.