The Mom Test
Ask questions that even your mom can't lie to you about.
Explanation
Rob Fitzpatrick's principle (adopted by YC): when validating ideas, ask about past behavior and current problems instead of future intentions. People lie about what they'll do in the future (especially to spare your feelings), but they tell the truth about what they've already done and what currently frustrates them.
Real-World Example
Bad questions: 'Would you use an app that does X?' 'Is this a problem for you?' 'How much would you pay?' Good questions: 'How do you currently solve this?' 'What's the last thing you bought to address this?' 'Walk me through the last time this problem occurred.' The difference: past behavior vs future promises.
How to Apply
Prepare questions about past behavior before user interviews. Ask for stories, not opinions. When people say they love your idea, dig deeper: 'How have you tried to solve this before?' Look for signs of real pain: workarounds, money spent, time invested. If they haven't taken action on the problem, they won't take action on your solution.