Physics
The Double-Slit Experiment: Wave-Particle Duality in Action
Quick fact
When electrons are fired one at a time through a double slit, they gradually build up an interference pattern on the detector screen, as if each electron interferes with itself—a behavior impossible for a classical particle.
Why this is interesting
Shine a light through two tiny slits, and you'll see a pattern of stripes—but fire a single particle through, and it still creates stripes, as if it went through both slits at once. How can something be in two places at the same time?