Ask HN: What should I ask a startup CEO in an interview?

First some context: I'm currently happily employed as a programmer working primarily with Rails but I touch lots of other stuff in a typical week. I really like my job. It's at a stable company with around 700 people. The pay is adequate but not fantastic. The environment, while frantic at times, is awesome. I also get lots of smaller but very pleasant perks like a free gym and an amazing view of my city's downtown.

I got contacted by a recruiter last week regarding a job as a senior programmer at a local startup. 99% of the time I ignore recruiter spam, but several things jumped out at me: I'm almost perfectly qualified (on paper anyways), the listed pay is slightly more than 1.7 times my currently salary, and the job is located in a suburb that's closer to my house than my current job.

I did a phone interview and that went very well. They gave me a small take home project to do and apparently they were "very impressed" with what I turned in according to the CEO.

I've got an in person interview tomorrow with their CEO. Given the way things are going so far I'm not at all worried about what they'll ask me. What I am worried about however is: what should I ask them?

The company has been around for 5ish years, has 18 people, and has raised greater than 5 and less than 10 million in series B funding. Supposedly the job would have me doing more or less what I do now. The only difference is that I would be managing other programmers and instead of working on a wide variety of different projects I would be working on a small handful.

I've never worked at a company like that before. Normally I wouldn't be so worried about what to ask them, just about getting the job. But, I really do like where I'm at and I don't need a new job. If I'm going to switch companies I want to make sure I know they're the right fit for me.

tl;dr - What can I ask them that will help me determine if they're stable, have a long term plan, and are a good place to work?