Every quantum particle must be described both as an object and as a wave. This wave-particle duality was discovered by a French scientist, Louis de Broglie, who was awarded the Nobel Prize for it in 1929. This is probably the strangest notion of quantum physics.
Indeed, in the world around us, matter is either particle (a tennis ball for instance), or undulatory (a wave in the sea, sound or light). A particle can be described by its position, its speed, and its trajectory. When two particles meet each other, they collide and move on in different directions. Conversely, a wave that spreads and does not stay in a particular place can be characterized by its frequency, its phase, and its direction of propagation. When two waves meet, they juxtapose and form a new wave. Sometimes they can even cancel each other if the top of one wave meets the bottom of the other one.
In quantum physics, both phenomena happen at the same time! For instance, an electron has a mass and an electric charge. When sent on a screen, its impact can be measured as if it was a bullet shot on the screen. But if you let the electron move freely without measuring it, it will have the characteristics of a wave and will spread in space in several places at the same time.
