What is foil on door knob?

Foil on a door knob is a simple technique some people use to determine if someone has entered a room or opened a door without their knowledge. To use this technique, a small piece of aluminum foil is wrapped around the base of the door knob. If someone touches the knob and turns it, the foil will become crumpled or even fall off, indicating that someone has entered the room. This method is often used in shared living spaces or hotel rooms where the occupant wants to know if another person has entered without their permission. However, it is not a foolproof method and may not work in cases where the intruder is careful and avoids touching the knob or uses gloves to hide their fingerprints.