Successful businesses start by solving big problems for an audience willing to pay. The author shares their experience co-founding an investing newsletter that grew quickly due to validating its idea on Reddit, and highlights the importance of identifying problems worth solving. To do this, ask yourself if the problem is situated in a growing market, if customers experiencing it have purchasing power, and if you're able and equipped to solve it. The author suggests finding problems by reading 1- to 3-star product reviews, niche Facebook Groups, subreddit data, and your own personal experiences. To validate a solution using Reddit, create a basic landing page, craft relevant posts, offer value, and get feedback from the community. This process can help you determine if an idea is worth pursuing or if you need to pivot.