Well, the short answer is the cells actually respond to changes in the chemicals in the molecules that exist on the outside of the cell. And these chemicals, these molecules can actually influence the processes that the cells carry out. And these processes ultimately lead to physiological responses such as running away from that hungry bear. So chemical changes in the environment around cells can influence the cells to carry out processes that ultimately lead to specific physiological responses. Now, beginning with that signal molecule that increases in concentration around that cell and ending all the way with that physiological response all the events that take place between these two points of time, this is what we call a signal transduction pathway. Transduction simply means we're passing that, that we're passing down that signal from one area to another area and we'll see exactly what that means in just a moment.