Right out of college I took a job selling commercial real estate. It was a situation where I knew nothing and brought very little to the table, but for whatever reason they decided to take a chance on me. I remember walking into my boss' office in the afternoons to give him updates on what I had been working on. We would talk through the status of deals and what needed to happen next. Inevitably, as I explained what I thought a particular buyer or seller was going to do or what I thought the price of a property should be or how we should advise our client, he would stop me. Being stopped was not typically a good sign and I quickly began to realize what was coming next. One question... "how do you know that"? It's a simple question and if you make a statement about something as if it's fact, you should know why it's true, right? The problem was my truth wasn't based on facts. I wasn't researching my hypothesis with data that was freely available or asking enough questions to ensure I understood. I was assuming and assuming can be lethal. 

To this day, it's still one of the most important lessons I've ever learned. The remedy is research and experimentation. It's better to research what other comparable items are selling for than to guess what people will pay. Better to experiment with a business idea on a small scale than launch a full blown operation based on a hunch. And it doesn't just apply to business. How many times have you assumed you knew what your spouse was going to say rather than just listening? How often are you positive you know the answer when all you've done is guess?