Fire single particles through two slits, and they still build an interference pattern

Light, and later electrons, passed one by one through two narrow slits produce the banded pattern you'd expect from overlapping waves — as if each particle interferes with itself. Watching which slit it goes through destroys the pattern entirely, a result still considered one of the strangest in physics.

— Thomas Young (1801), later extended to electrons by Davisson, Germer and others, Double-slit experiment — Original light experiment, 1801; electron version, 1920s–70s