Many problems that arise in making egg salad, chopped liver, tuna fish, etc. (Besides the issue of LOSH).
Understanding the concept of מלאכה שאין צריכה לגופה;
(Squeezing materials.) understanding the two different types of "סחיטה" (squeezing);
Understanding "פסיק רישא" (inevitable results from one's permissible action).