49 Who vs. That

This is extremely minor, but it’s a pet peeve of mine, so I will mention it.  Who as a relative pronoun is used when referring to human beings; that as a relative pronoun is used when referring to things that aren’t human beings.  Thus it is better to write, “I had a conversation with a scientist who thought grammar was more interesting than physics” than “I had a conversation with a scientist that thought grammar was more interesting than physics.”  (This is assuming the scientist is a human being, of course…)