A person can be expert in nothing, but they must be practiced in many things. You simply have to collect and develop a certain number of skills as the years tick by. People count on you to come through.
All too often, it can be easy to stick with the things we're comfortable with. It's much easier to keep going to job we know (even if we don't like it at all), than it is take the risk of moving onto a new one. But taking risks (calculated ones), is neces