When students learn to program, they are often worried about breaking something. Explaining what can and can’t break the computer can be empowering for students. Also, students are often allowed to drink water during class, so they might not understand why they can’t have any food or drinks around the computers.
Example: "We have to be careful because water could ruin the computer."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-lab-rules