A triangular pyramid, also called a tetrahedron, has a triangular base with three triangular faces rising from it. How many faces does it have in total?

Erklärung

The three sloping surfaces are closed off by the triangle underneath, and that base is a face just like the others, so three and one are counted together. Answering 3 leaves out the surface the solid would stand on, which is still part of its boundary.