Understanding the concept of "מדידה של מצוה" (measuring for a mitzvah).
(Temporary Ohel) Some more issues in reference to umbrellas on Shabbos;
Pushing Out a Dented Water Bottle, Using Toys That Are Made to Deflate and Put Away, Putting Back a Nail in the Shoe, and...
Is one allowed to knock on the door with the key, knock on the window, use an electric bell, and have a little bell...