Stoikov market-making model: optimal quoting in one page

You're about to play a market-making round on this site. Before you do, spend 4 minutes reading the framework most quant interviewers will quiz you on: Avellaneda & Stoikov (2008).

This is a primer, not a derivation — no questions, just the four intuitions you need to walk into the interview and explain why a market maker's quotes are NOT centred on the mid-price.