A graph is a thing made of point, some of which are linked by line segments. Generalize the idea to points that can also be grouped into triangles, or tetrahedra, etc.
For graphs we know Euler's formula E+2=V+F. Give this a geometric meaning.
Discuss (maybe in the 2-dimensional case) what might replace this formula compare a "triangulated" sphere to a "triangulated" doughnut.
