One who writes, although he is not liable according to the Torah but only in the case when writes with something permanent over a permanent thing (like engraving letters over stone), in all other cases it is still forbidding according to the rabbis, even if he writes with something not permanent over something not permanent (writing on snow with your finger). Therefore, it is forbidden to write with your finger using liquid over the table, or to leave an impression on the dust which is on the glass / cristal, or to leave an impression with the nails over a piece of paper, and also one should be careful not to leave an impression with a nail over a book in order to remember something, and even if he did leave an impression with no writing form whatsoever, this is still forbidden.