In this paper we discuss some basic concepts of knowledge engineering, expressing our feeling that there is something missing. In our opinion, the missing link, able to clarify all the basic concepts, should be the concept of energy, not in use in knowledge engineering practice and theory at all. Since energy is an extremely abstract concept, which led to misunderstandings even in physics, we humbly introduce it by describing an existing expert system, called Hascheck. Hascheck is a spell checker for Croatian language, implemented as learning (semi)automaton, accessible via E-mail and Web, and being in public use nearly 10 years. We also present here a mathematical model of Hascheck's learning. The model led to the so-called cognoelectrical analogy, which allowed some energy considerations about learning, and knowledge.