Understanding the type of knots that are allowed to be opened on Shabbos.
Eating Hard Food When It Might Make One's Gums Bleed, and Removing Meat Stuck in One's Teeth When Gums Might Bleed;
What are the circumstances needed to consider one's actions "מדידה של מצוה" (measuring for a mitzvah).
pull tab rings attached to orange juice containers, grapefruit juice containers, etc. (Making a nice usable opening on Shabbos.);