Magnets hold notes to a fridge because steel in the door attracts the magnet strongly enough.

The longer version

Refrigerator doors are steel, and steel is drawn strongly to a magnet because its internal domains line up with the field and are pulled toward it. The magnet grips the door through that attraction, and friction between magnet and door resists the downward pull of the paper, so a small magnet holds a surprisingly heavy note.

Fridge magnets are made from many narrow strips of alternating polarity, which concentrates the field on the front face so the magnet sticks hard on one side and barely at all on the other. Stainless doors often will not hold magnets, since some stainless alloys respond far less, which surprises people after moving house.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.