It states that the active side is not exactly a perfect fit. However, as the substrate basically goes into the active side what happens is the active side, the actual enzymes active side as well as the substrate itself actually change in shape ever so slightly to basically create that perfect fit. And that makes sense because if we examine the subatomic level, if we examine the molecular level and we discuss the electric forces involved whenever we have the interaction between different molecules these electric attractive and repulsive forces will basically change the shape of the molecule just a little bit. And this can be basically explained from a physics perspective. So basically we can imagine our enzyme initially has a rectangular active side while our substrate is, let's say, some type of trapezoid. Now, when the trapezoid, the substrate shown a red actually goes into this rectangular active side.