The rate of change of an exponential is not constant. But there is a linear relation between its derivative (=rate of change) and the actual function. Or more explicitely d(e<sup>t</sup>)/dt=e<sup>t</sup> , so when you take the derivative you get the same function back. Just like when you take the function y=x insert a number for x then the output, y, is equal to x. But in the case of the exponent the function in the previous example gets swapped with the derivative.