In our everyday lives we are constantly coming into contact with an endless variety of things, in our homes, in our journeys to and from school, at work, or at play - books, pencils, marbles and the list can be added to indefinitely. We can classify them in any way we please, by weight, by colour, or by age, but in spite of the endless diversity of the objects we have listed, there is an important property that they all possess. Each takes up a certain amount of room or space.
Anything which takes up space is spoken of as a solid.
Thus each page of our books is a solid, however thin the paper may be. The air we breathe and the water we use have also the property of occupying space and are therefore solids. The word solid as used here must not be confused with the word solid which is used as opposed to liquid and gas.
Most solids are irregular in shape, e. g. a pebble in a stream, a cloud in the sky. Geometry deals with the shape, size, and position of solids which are regular in shape, e. g. a ball, a match-box, a pencil.
The more common regular solids are: cube, cuboid or rectangular prism, triangular prism, square pyramid, cylinder, cone, sphere.