The Torah prohibits melakha on Shabbat only when the result is lasting (mitkayem). If it is not lasting, there is no Torah prohibition. Therefore, one who writes with a pencil or pen on paper transgresses a Torah prohibition, since this writing will last for a long time. However, if one writes with fruit juice whose color fades quickly, or on a leaf that will soon dry out and crumble, then the prohibition is rabbinic. Similarly, writing in sand or in the condensation on a window is rabbinically prohibited since it does not last (below 18:2, 4).
Along the same lines, one who ties a durable knot that will hold for an extended period of time transgresses a Torah prohibition, while one who ties a temporary knot that will not last long transgresses a rabbinic prohibition. A very loose knot that is easily undone and has absolutely no permanence, like a single knot or the knot of a tie, is not prohibited at all, since it is easily untied (below 13:13). The same applies to the melakha of Boneh: if one affixes a hook to the wall in a permanent way, he transgresses a Torah prohibition, while one who affixes it temporarily transgresses a rabbinic prohibition (below 15:3).
