Within (2) What always becomes, our realm of sensation, Plato says it is most probable that everything is made out of triangles. This is at the basis of his geometry that Epíkouros attacks, but it is the basis for some other absurd ideas: triangles in your stomach and blood breakdown food, this becomes less efficient as you get older -- because your triangles are getting increasingly more dull (81c), this is, in fact, exactly what aging is.
The elements are built from and can be broken back down into triangles. If it seems to you that triangles alone might not be sufficient as a basis for everything we see in the word -- you have not considered that these triangles have different angles and come in different sizes (57d).