Shu and I were planning to meet at the Art Institute in Chicago, and he forgot his cell phone. Therefore, he called from a pay phone and left a message for me to meet him at the top of the stairs. I went to the outside stairs and waited, and he was waiting on the inside stairs. Eventually we met, but here is the question:

He said I should have known that "stairs" are inside, and that "steps" are outside. I didn't know that. However, in looking that up, I don't see that he is right. The dictionary says that a stairs is the flight of steps.

How do you use these two words?

I would normally use 'steps' to refer to an outside flight, and 'stairs' to mean one inside a building. They are fairly interchangeable, though, and I might sometimes refer to an individual stair as a 'step'.