Quantum objects exhibit both wave and particle properties depending on how they are observed — this is wave-particle duality. Photons interfere like waves in a double-slit experiment yet arrive at the detector as discrete clicks. Electrons show the same interference pattern when no attempt is made to determine which slit they passed through, but the interference vanishes when their path is measured. Bohr's complementarity principle states that wave and particle aspects are mutually exclusive experimental situations, not contradictory properties of a single object.
Build intuition by considering the double-slit experiment for photons, then electrons, then molecules. Focus on what changes when a 'which-path' detector is introduced and why the interference disappears.
By the early 20th century, two experimental results had shattered classical physics from opposite directions. The photoelectric effect (Einstein, 1905) showed that light — long established as a wave — delivers energy in discrete packets called photons, with each packet's energy proportional to frequency. Meanwhile, electron diffraction experiments showed that electrons — clearly particles with definite mass and charge — produce the same kind of interference fringes as X-ray waves passing through a crystal. The same kinds of entities were behaving as both waves and particles, depending on the experiment. This is wave-particle duality.
The double-slit experiment makes the paradox vivid. Fire electrons one at a time at a screen with two narrow slits: each electron produces a single localized click on the detector behind — unmistakably particle-like. Watch where those clicks accumulate over thousands of electrons, and an interference pattern builds up: alternating bright and dark bands that can only be explained if each electron's probability distribution was shaped by wave interference between paths through both slits. The electron does not split; its wavefunction — which encodes probabilities — spreads through both slits and interferes with itself. The detection event collapses that wavefunction to a point.
The deepest part of the story is what happens when you try to resolve the paradox by watching which slit each electron uses. Place a detector at the slits to record the path — and the interference pattern immediately vanishes, leaving only two plain bands. No disturbance to the electron is required; the mere fact of which-path information being available in the environment destroys the interference. Bohr called this complementarity: wave behavior and particle behavior are mutually exclusive aspects of the same system. The experimental arrangement determines which you observe. You cannot observe both simultaneously.
Wave-particle duality is sometimes misread as uncertainty about what the electron "really is." A more accurate framing is that quantum objects are neither classical waves nor classical particles — they are quantum systems described by wavefunctions. The wavefunction evolves according to a wave equation (which is why interference occurs) but yields definite, localized outcomes upon measurement (which is why detectors click at specific points). The apparent contradiction dissolves when you stop demanding that quantum objects conform to the categories of classical physics.
One important correction to the popular picture: duality is not limited to fundamental particles. Interference has been demonstrated for fullerene molecules (C₆₀, "buckyballs") and molecules containing hundreds of atoms. The reason we do not see interference for billiard balls or baseballs is not a sharp size boundary but a process called decoherence — rapid, unavoidable entanglement with the environment that destroys quantum coherence. Duality is universal in principle; decoherence is what hides it at everyday scales. Understanding duality is the prerequisite for the wavefunction interpretation, the uncertainty principle, and every subsequent concept in quantum mechanics.