"Ramen Profitable" ~ http://www.paulgraham.com/ramenprofitable.html and "Ideas for Startups" ~ http://www.paulgraham.com/ideas.html
At any given time there tends to be one problem that's the most urgent for a startup. This is what you think about as you fall asleep at night and when you take a shower in the morning. And when you start raising money, that becomes the problem you think about. You only take one shower in the morning, and if you're thinking about investors during it, then you're not thinking about the product.