It lightens the atmosphere and encourages students to attend.
Pizza is easy to order and serve, and almost everyone enjoys it.
As an undergraduate student training other undergraduate students, one needs to maintain some formality to ensure credibility. During a two-hour workshop crammed into a day already full of boring classes, it's easy to lose students' attention, even in a small workshop setting. If anyone appears to doze off, suggest a five-minute break, or quickly wrap up the current discussion and move on to something else. Because students do not have many opportunities to discuss their learning, they may become excited by the discussion topics and jump to discussing experiences irrelevant to the discussion. Intercept these tangents as quickly as possible, saying that while they interest you and are valuable, they are not relevant to the current discussion.