The world is made of invisibly small particles of various shapes and characteristics. They come in a few kinds which correspond to the following '''elements''': ''earth, water, air'' and ''fire''. A single particle of each element has the shape of a regular convex polyhedron (also known as a Platonic solid). The particles of ''earth'' are cube-shaped, cubes being the only Platonic solids which can be arranged to fill space completely, which is why ''earth'' comes in solid phase. ''Water'' particles are little icosahedra (20-faced solids), which are most similar to a sphere, which is what makes water run smoothly through your fingers. ''Fire'' burns because its particles have the form of a (four-faced) tetrahedron with pointy vertices. A particle of ''air'', which is also gaseous, but not as light as ''fire'', is an octahedron, which has eight faces, or twice as many as a ''fire'' particle.
The problem is that there are exactly five regular convex polyhedra (which was mathematically proven already in antiquity). Which means there must exist a fifth element (also known as "quintessence", its Latin name), which would correspond to the fifth Platonic solid, the (12-faced) dodecahedron. In the far future, Luc Besson will imagine the fifth element to take the form of Ms. Milla Jovovich's supple body, but ancient natural philosophers thought it was actually ''aether'', a very subtle substance of which heavenly bodies are made. As ''aether'' does not occur on Earth, we won't be paying any more attention to it.
The remaining four elements may be described by a pair of characteristics: ''hot/cold'' and ''moist/dry''. And so, ''earth'' is ''cold'' and ''dry''; ''water'' is also ''cold'', but ''moist''; ''air'' is ''moist'', but ''hot''; and ''fire'' is, obviously, ''hot'' and ''dry''. Every single thing, substance and living being is composed of particles of these four elements in various proportions, which determines each thing's own '''complexion'''. Something which contains, say, more particles of ''fire'' and fewer particles of ''water'' is of ''hot'' and ''dry'' complexion – in a higher or lower degree, depending on the dominance of ''fire'' in the thing's elemental composition.