As we have seen, temporary writing is rabbinically prohibited. Erasing something when one does not plan to write something else in its place is rabbinically prohibited as well. Therefore, one may not write in the condensation on a window or erase such writing. Similarly, one may not form letters in the sand or erase them (MB 340:20-21).
One may not mark a page using his fingernail to remind oneself that the page contains something notable or something that needs to be corrected. Since he is creating a lasting mark, it is rabbinically prohibited (MB 340:25; Kaf Ha-ĥayim ad loc. 51). However, one may dog-ear the page, because there his goal is not to “engrave” a mark in the paper. Rather, the fold itself is the mark.
Some allow a sick person to use a thermometer strip. They maintain that it is not really writing since the numbers are already imprinted on the strip, and the temperature only makes them visible for a short time, after which they disappear (Yeĥaveh Da’at 4:29). Others forbid this, considering it temporary writing (SSK 40:2). Since the issue is rabbinic, one may be lenient in times of need (Tzitz Eliezer 14:30; below 28:11). Similarly, le-khatĥila one should not perform any medical tests that cause colors to appear, but in times of need, one may be lenient (ibid.).
Colorful blocks may be put next to each other to form a letter, and thread may be laid down in the shape of a letter. This is because the blocks or thread already exist; one is simply arranging them. Similarly, he may juxtapose two cards that together form a picture, letter, or word. This is because all the forms already exist; they are simply being brought together.
This applies when the different parts do not connect to each other or to a board. However, if they do connect, most contemporary poskim maintain that it is rabbinically forbidden to put them together and that therefore one may not pin parts of letters to a board to form a whole letter, on account of Kotev. Similarly, one may not assemble a jigsaw puzzle, since creating meaningful pictures also constitutes Kotev. Others permit these activities, maintaining that they are not considered writing since all the writing already exists, and the activity merely brings the letters or puzzle pieces into proximity. However, even according to the lenient position, one may not complete a jigsaw puzzle in order to preserve it as a portrait.
In practice, those who wish may be lenient and allow their children to engage in these activities, but it is proper for adults to be stringent, as most poskim are stringent here. More generally, it is important to realize that Shabbat is meant to be dedicated to Torah, and some therefore say that adults may not play games on Shabbat at all (below 22:13, 24:7; and Harĥavot).
One may use a multiple-dial combination lock on Shabbat, because turning the discs to align the numbers is not considered writing. The numbers are already there; one is merely realigning them temporarily to form the correct combination and open the lock (Tzitz Eliezer 13:44).