What if you could buy stocks with your smartphone—and avoid annoying trading charges to boot? That’s how millions of young investors use Robinhood. Bhatt and Tenev, who met as Stanford math whizzes, launched the company in 2013 as an egalitarian challenge to oldguard brokerage houses. Five years later, it has shot to a valuation of $5.6 billion. This year they added Bitcoin and other cryptocurrencies to the trading mix and now hope to build Robinhood into a full-blown digital bank.