Understanding the type of knots that are allowed to be opened on Shabbos.
הכנה ליום אחר (Preparations for a Weekday): Washing Dishes on Shabbos for Sunday. Releasing a Halachic Vow (הַתָּרַת נְדָרִים);
opening boxes with perforated openings, soda can boxes, salt and sugar containers;
Various Halachas Pertaining to Making a Salad on Shabbos;