“What, then, is the difference between a tool (or toolkit) and a medium?

The difference is [..] that the tool does what we want, amplifying, in some sense, our natural abilities. It is applicable within the field of its intention. The medium, on the other hand, ‘bites back’. It suggests other uses, may not quite work as we want (not be a terribly good tool), will come to inform, even dominate our use of it — giving us new, unexpected and unintended uses, so that using it becomes an interaction, and we are affected by the medium as much as we affect it.”