English: In this experiment, a beam of individual electrons (or other quantum particles) is fired one after another at a barrier containing two narrow slits. On the other side of the slits, a sensitive detector registers where an electron struck.

If electrons behave like particles---imagine tiny billiard balls---you would expect to see a pattern of two clusters at the detector, with one cluster behind each slit. Instead, the detector records an interference pattern, as though each electron traveled through the two slits as a wave.

A wave passes through both slits at the same time. The resulting interference creates a series of bright and dark spots on the wall.

It's an experiment that challenges intuition, but quantum physics explains what's going on. The two slits expose the wavelike behavior of individual electrons by channeling each of them into a quantum superposition of "went through the left slit" and "went through the right slit." The wave patterns emanating from the two slits interfere with each other, ultimately yielding the pattern on the detector.