I've got a regular tetrahedron and a square pyramid. Every edge of the two solids has the same length. If I perfectly attach one face of the tetrahedron to one of the triangular faces of the square pyramid (I.E. every point of one face overlaps a point of the other face, edges and vertexes included) how many faces will the new solid have?
Edit, let me clarify: this only has to do with geometry. No lateral-thinking, no word puns, no silly explanations (otherwise I would have added some of these tags), just pure and simple geometry. Yes, it may look stupid, but it isn't.
Inspired by "How many faces does the resulting polyhedron have?" on Math SE.