Question. Context: When unspecified, a pyramid is usually assumed to be a regular square pyramid, like the physical pyramid structures. A triangle-based pyramid is more often called a tetrahedron.

Question: what do you call a pyramid with a triangular base?
Answer. a tetrahedron
What step-by-step reasoning justifies that answer?
Reasoning: The relevant sentence in the passage is: A triangle-based pyramid is more often called a tetrahedron.