One of the key experiments in physics is called the double slit experiment. The experiment very elegantly demonstrates why quantum physics is so counter intuitive. The double slit experiment shows that light is both particles and waves. It also tells us that electrons, subatomic particles themselves, can behave like waves. This so-called wave-particle duality is one of the key concepts in quantum physics.

Today's video is a cool cartoon that explains this concept and describes the experiment in simple terms. It really is one of the best explanations I've seen for the subject.

Finally, if you want a reference to dive into further, check out this Physics World article.