When I first walked onto a trading floor in 1989 the big thing was airline takeovers. They were the hot stocks of the day and even DJT got in the action with making a bid for American Airlines. Since then I have seen biotech, nanotech, tech, internet, 3d, emerging markets, etc, etc…. as the latest hot thing. You know when something is hot when a company goes public on the just the idea that they are going into an industry. A mini-bubble can happen within a bull market and can come and go with barely a ripple. Now we have Bitcoin (BTC).