Every quarter, investment firms that manage more than $100 million file a form 13F with the Securities and Exchange Commission (SEC). I find the 13F to be a valuable tool because it breaks down in detail which stocks institutional investors are buying and selling, and it can be interesting to try and identify patterns among Wall Street's biggest money managers. One investor that I enjoy following is Jeff Yass, the co-founder of Susquehanna International Group (SIG).