How can we help individuals handle financial decisions in an increasingly complex environment? We explore an easily scalable avenue for improving financial understanding: learning by online trading in stocks. We randomly assign 1345 adults incentives and opportunities to trade stocks for 4-7 weeks, with no additional educational content. The treatment significantly improves financial literacy and attenuates the gender gap in self-assessed financial knowledge. Treated individuals are more likely to subsequently invest in stocks and less likely to seek external advice. The effects strengthen for those exposed to index funds, foreign assets, and rising or more volatile asset prices.