The search for the smallest particle of matter led scientists to develop new observation tools, such as very powerful microscopes. Scientific observations using these new tools led to the discovery of small paticles called atoms. All matter is made up of these small particles or atoms. Atoms are always moving, but the movement of one atom is very hard to observe, because it is so small. Atoms are made up of smaller particles that are also in motion.
Scientists have difficulty observing the actual movement of the particles that atoms are made of.
Therefore, particle movement is more easily observed by noting changes caused by such motion. For example, while air cannot be seen, it often can be used to show particle motion. Place an open bottle of perfume at one corner of a room. Get a friend to stand at the opposite corner of the room and time how long it takes for them to first smell the perfume. This shows that the perfume particles move along with the air particles.