A further physical observation, known as Brownian motion, suggests that matter may be
composed of tiny moving objects. When tiny dust or smoke particles are placed in a gas or
liquid and observed under a microscope, they are found to move around in a random manner.
They move first in one way, and then suddenly change direction in a random, haphazard way.



We can begin to answer this question by considering several simple observations that
suggest matter consists of particles. One of the most beautiful of these is the existence of
crystals. Under certain conditions, a solid may form into wonderful geometric shapes, and the
shape is always the same for a given substance. The simplest way to explain this phenomenon is
to imagine these crystals to be composed of a tight packing of identical "building blocks." The
shape of these individual building blocks might then be the cause of the crystal shape.



Another suggestion for the existence of basic building blocks of matter is a simple
observation of certain oil drops on water. When a small amount of such an oil is placed on
water, it will spread out until it reaches a very definite area, and then remain at this size. This
shows that the oil can be made to spread only so thin before it must break into separate drops.
One explanation is that the oil is composed of tiny identical building blocks which will spread
out until they lie just one building-block thick.