"Liquidity" is one of the most widely-talked about yet least understood concepts in markets. Roughly speaking, a market is liquid if you can transact in it without affecting the price significantly. But there's little agreement about why some markets are more liquid than others, or why liquidity sometimes just evaporates with little notice. This week we speak to Karthik Shashidhar, the author of "Between The Buyer And The Seller" about what we can learn about liquidity from things like Uber, dating apps, and real estate brokers.