What is foil on doorknob for safety?

Foil on doorknob is a safety technique used to identify if someone has entered a room or home when they were not authorized. Foil can be placed on a doorknob in such a way that it is difficult to detect at first glance. If someone tries to enter the room, the foil makes noise and moves, indicating the non-authorized entry. This technique is often used by people who live alone, in dorm rooms, or when staying in hotels. It is a simple and effective way to detect unauthorized entry and can help prevent break-ins and theft. However, it is important to note that foil on a doorknob should not be relied upon as the sole defense against intrusion. It is always recommended to have proper locks and security systems in place in addition to any DIY safety measures.