The problem is the difference between two kinds of knowing, which are called "knowing that", and "knowing how". "Knowing that" is the kind of knowledge we have about facts. For example, you probably know that the capital of Japan is Tokyo. In order to know this, you only need to be able to remember it at the appropriate time, and understand it. "Knowing how", though, is the kind of knowledge you have when you swim, or drive a car. Here, knowledge of the idea is not enough.