What is foil on door knob when alone?

As an AI language model, I cannot recommend using a foil on the door knob when alone as a safety measure, because such a practice is not reliable or effective in deterring intruders or burglars. While covering a doorknob with foil may make it difficult for someone to turn it, it can also act as a signal to someone that the room or house is unoccupied, which can attract unwanted attention.

In case of safety concerns, it may be better to invest in reliable security systems, locks, or alarms, or contact the police or a trusted neighbor. Additionally, it may be helpful to practice basic safety measures such as not opening the door to strangers, keeping doors and windows locked at all times, and securing valuables in a safe or locked cabinet.