In the early 1900s, many scientists thought that matter was continuous and could always be divided into smaller pieces. Some scientists disagreed and thought that matter was made of lots of small particles, called molecules. Einstein was one of the scientists that believed matter was made up of molecules.
In 1905, when he was 26 years old, Einstein studied “Brownian motion.” Small particles, like dust, move around randomly when they float in a liquid (Random Walk). You can see this if you look at water under a microscope. This movement is called Brownian motion, and before 1905 no one knew what caused it.
Einstein realized that if liquid was made of small molecules that were always moving around, this would explain Brownian motion. The molecules, he figured, would sometimes bump into the dust particles and cause them to move in different directions-- like what happens in bumper cars. Einstein used math to predict how the dust should move, and he was exactly right. He also figured out how to measure the size of molecules (Caliper with Atom).
Einstein’s work showed scientists that matter is made of molecules (Magnifying Glass). Since 1905, we have learned lots more about the molecules and atoms that make up different kinds of materials (Crystal,Superconductivity). This work has many applications in technology (STM, Computer Chip), and is still an important area of research (Quantum Corral).