If you get one trading interview question, it is this one. An interviewer asks for a market on something you cannot possibly know exactly — and then trades against your answer.
Start the Make Me a Market drillFour other drills are free to try first"Make me a market on the height of Mount Everest" is not a geography question. You are being asked to state two numbers: a bid, at which you would buy, and an ask, at which you would sell. The interviewer then chooses one and trades against you.
The trap is that both errors are punished. Quote too wide and you look cowardly and nobody trades — a market maker who never trades earns nothing. Quote too tight around a bad estimate and you get picked off by someone who knows more than you.
Estimate first, then choose a width. Decide your honest midpoint before thinking about the spread. Candidates who pick a spread first end up anchoring their estimate to it.
Let your width express your uncertainty. A market on the number of US states should be very tight. A market on the length of the Nile should be wide, because your uncertainty is genuinely large. Interviewers are reading whether your confidence is calibrated.
Update when you get traded. If someone lifts your ask immediately, that is information — they think it is worth more than you do. A good market maker shifts the next quote up. Candidates who ignore this signal are showing they will not learn from the market.
This drill runs three rounds per fact. The first allows any width — your honest confidence interval. The second caps the spread at 30% of your previous midpoint, the third at 12%.
That escalation mirrors the real interview, where an interviewer says "tighter" until you are uncomfortable. It is deliberate: they want to see where your composure breaks and whether you start quoting nonsense.
Roughly half the order flow here knows approximately where the truth is. That is what makes the exercise real — against random flow, any quote is fine on average. Against informed flow, a lazy midpoint bleeds money.
The result screen shows every trade and whether you earned the spread or were picked off. There are 334 facts across ten categories, so the drill does not run out.