Paul Graham has watched plenty of start-ups succeed. He’s also seen some fail. He’s the co-founder of Y Combinator, the Silicon Valley tech incubator. Graham has an extraordinary track record, including success stories like Dropbox, Airbnb, and Reddit.

By helping to grow hundreds of start-ups, Graham has had the opportunity to figure out the secret of success. Turns out it’s common sense.

The secret, according to Graham, is “Make something people want.” If you’re not succeeding, it’s probably because you’re not making something people want.

Graham uses restaurants as an example. If a restaurant has really good food, it can be in an obscure location, charge a lot, and have really bad service and still be popular. I know that’s true. My wife has dragged me to the middle of nowhere to eat in places that make me worry about sanitation. The service in some of these places has been nearly nonexistent. The food has been fantastic. People are lined up around the block.

“Make something people want” applies to law firms just as much as to tech start-ups. If you’re not succeeding, it’s likely because you’re not making something people want.