The square sandwich is an item used in the 20th Anniversary celebrations event. It is obtained from the sandwich lady for free during the miniquest for top quality milk, and is given to Thok in exchange for his big rock.
The sandwich lady has only made enough sandwiches for one each, so having destroyed or eaten the sandwich results in her reluctantly giving you another, stating that someone else doesn't get a sandwich now.
Attempting to eat the sandwich will trigger a message saying that you should not eat it because you are going to give it to Thok. Refusing to eat it, your stomach rumbles, and eating it anyway causes the player to have to get another from sandwich lady.
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