Complex sentence: The origin of the water part of a watermark can be found back when a watermark was something that only existed in paper, at that time the watermark was created by changing the thickness of the paper and thereby creating a shadow/lightness in the watermarked paper.
Split into simpler sentences: The origin of the water part of a watermark can be found back when a watermark was something that only existed in paper. At that time the watermark was created by changing the thickness of the paper and thereby creating a shadow/lightness in the watermarked paper.