One of the first things we ask YC companies is what they’re building and why.

We look for clear, concise answers here. This is both to evaluate you as a founder and the idea itself. It’s important to be able to think and communicate clearly as a founder—you’ll need it for recruiting, raising money, selling, etc. Ideas in general need to be clear to spread, and complex ideas are almost always a sign of muddled thinking or a made up problem. If the idea does not really excite at least some people the first time they hear it, that’s bad.
Another thing we ask is who desperately needs the product.
In the best case, you yourself are the target user. In the second best case, you understand the target user extremely well.

If a company already has users, we ask how many and how fast that number is growing. We try to figure out why it’s not growing faster, and we especially try to figure out if users really love the product. Usually this means they’re telling their friends to use the product without prompting from the company. We also ask if the company is generating revenue, and if not, why not.
If the company doesn’t yet have users, we try to figure out the minimum thing to build first to test the hypothesis—i.e., if we work backwards from the perfect experience, we try to figure out what kernel to start with.

The way to test an idea is to either launch it and see what happens or try to sell it (e.g. try to get a letter of intent before you write a line of code.) The former works better for consumer ideas (users may tell you they will use it, but in practice it won’t cut through the clutter) and the latter works better for enterprise ideas (if a company tells you they will buy something, then go build it.) Specifically, if you are an enterprise company, one of the first questions we’ll ask you is if you have a letter of intent from a customer saying they’ll buy what you’re building. For most biotech and hard tech companies, the way to test an idea is to first talk to potential customers and then figure out the smallest subset of the technology you can build first.

It’s important to let your idea evolve as you get feedback from users. And it’s critical you understand your users really well—you need this to evaluate an idea, build a great product, and build a great company.

As mentioned earlier, startups are really hard. They take a very long time, and consistent intense effort. The founders and employees need to have a shared sense of …