I teach seventh grade math, and we also have had the debate in my classroom with my students on whether or not a cone has one face to two faces. The fact is that a cone has two faces. The definition of a face being a flat side comes from the net of a shape. When you look at the net of a cone, it has one flat circle and one flat sector (triangle with a curve). That being said, a cone has two faces and not just one. That should help a bit. When thinking about 3D shapes it is good to take a look at the net to find out how many faces a shape has. Check out this link:
http://www.mathsteacher.com.au/year8...s/cylinder.htm