How do you know people want this?

What YC looks for, above all, is founders who understand some group of users and can make what they want. This is so important that it’s YC’s motto: “Make something people want.”
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Who are your first users going to be, and how do you know they want this? If I had to decide whether to fund startups based on a single question, it would be “How do you know people want this?”

The most convincing answer is “Because we and our friends want it.” It’s even better when this is followed by the news that you’ve already built a prototype, and even though it’s very crude, your friends are using it, and it’s spreading by word of mouth. If you can say that and you’re not lying, the partners will switch from default no to default yes. Meaning you’re in unless there’s some other disqualifying flaw.