How to do it
Talk about their life, not your idea
The moment you describe what you are building, the conversation becomes a performance of politeness. Keep the idea out of the room for as long as you can — ideally the entire first conversation.
Ask about specifics in the past, never opinions about the future
'Would you pay for this?' is unanswerable; nobody knows. 'What did you pay for the last thing you bought to fix this?' has an answer, and the answer is a fact. Replace every hypothetical with a history question and the signal quality changes immediately.
Chase the concrete, especially numbers and dates
When someone says it is a huge problem, ask when it last happened, how long it took and what it cost. Vague enthusiasm collapses into either a real story or an admission that it is not that bad. Both are useful; only one of them would have survived a survey.
Treat compliments as a warning sign
'That sounds great' is the sound of a conversation that has stopped producing information. It usually means you started pitching. Steer back to their last experience and the signal returns.
Worked example
Why the rule is named after your mother
The premise is that your mother will tell you your idea is wonderful regardless of its merits, because she loves you. The insight is that this is not a problem with your mother — it is a problem with the question. Ask her what she did the last time she had the problem and she will answer accurately, because the question no longer requires her to have an opinion about you.
Every person you interview is doing a milder version of the same thing. Strangers are polite, customers do not want to discourage a founder, and anybody you have already pitched has an incentive to be encouraging. The rule works by removing the opportunity to be kind.
What to do when they ask what you are building
They will, usually about ten minutes in. The answer that preserves the conversation is a short, flat description with no enthusiasm attached — one sentence, then a question back about their own experience.
What ruins it is a pitch. Once you have described the vision, everything after is a response to you rather than a report of their life, and the remaining interview time is wasted on a conversation you could have had by email.
What goes wrong
- Describing the idea in the first two minutes and spending the rest collecting compliments
- Asking for a commitment before establishing that the problem exists
- Writing down the encouraging parts and discarding the hesitations
- Treating 'I would definitely use that' as a result rather than as a signal you pitched
Is your number plausible?
Each check catches a different way the arithmetic can be right and the answer still wrong.
- Read your notes: if most lines are about your product rather than their week, the conversation failed the test
- Count the facts you collected — dates, amounts, tool names. Fewer than three means you mostly heard opinions
- If nothing you heard was uncomfortable, you probably did not ask anything that could have gone badly
Related answers
Is the Mom Test a book?
Yes — a short one by Rob Fitzpatrick. The rule above is the whole of it applied; the book is worth an afternoon mainly for the transcripts, which show how fast a good conversation turns bad when the founder starts talking.
Does this work for consumer products too?
Yes, and arguably better. Consumers are even more prone to hypothetical enthusiasm than businesses, because nothing in the conversation has a budget attached to discipline it.
What if nobody has the problem often enough to remember?
That is a finding, and an important one. A problem nobody can date is a problem too infrequent to build a subscription business on — which you have now learned for the price of six conversations.