Understanding the concept of "מדידה של מצוה" (measuring for a mitzvah).
Making a "ROSHIM" (mark) on paper, on a page in a sefer.
Having a Non-Jew Sweep a Dirt Ground;
The Halachas of Shabbos When תִּשְׁעָה בְּאָב Falls Out on Motzi Shabbos;