Hi, Bot Machines · Fig. 02
Fridge light

How does it know the door is open?

It has a tiny magnetic switch and one rule. Open or close the door, then trick the reed with a magnet.

A fridge with its door and interior light.
Door
Fool it
Senses
Rule
Does

For grown-ups: what's really inside

Many fridge doors hide a magnet. On the frame sits a reed switch — two thin metal contacts that close when a magnet is near. Door shut: magnet near, reed closed, light OFF. Door open: magnet away, reed open, light ON. The machine never sees a door; it only feels that magnetic field.

Simplified: holding a magnet away from the reed (or covering it so the door magnet cannot reach) makes the switch think the door is open even when it is shut.