In the Timaeus, Plato uses the geometrical shape of each element to explain its appearance and behavior (tr. Jowett, revised):

First, let us ask what we mean when we say that fire is hot. We get a clue from the divisive and cutting effect it has on our bodies. We all feel that fire is sharp, and we should also consider the sharpness of the sides [of the four triangles that compose fire]...which make fire cut whatever it touches. We should also remember that the original [pyramidal] shape of fire. more than any other form, has a divisive ability to cut our body into small pieces. This naturally causes the sensation called heat...

We give earth the form a a cube, since earth is the most immovable of the four elements and the most malleable of all bodies­and whatever has this nature must necessarily also have the most stable basis. Now, of the two kinds of triangles...the one with two equal sides [i.e. the equilateral triangle] naturally has a firmer basis than the kind with two unequal sides [the isosceles triangle]. The cube of earth, then, is made of this former kind of triangle...

When earth encounters fire and is dissolved by its sharpness...it scatters here and there until its parts meet and blend together again and become earth once more. However, when water forms again after having first been divided by fire or air, may become one part fire and two parts air; and a single volume of air can be divided into two parts of fire...