Here's why:

* Speed is the rate of change of an object's position over time. It is a scalar quantity, meaning it only has magnitude (like 10 meters per second).

* Acceleration is the rate of change of an object's *velocity* over time. Velocity is a vector quantity, meaning it has both magnitude and direction (like 10 meters per second east).

* Since speed is the magnitude of velocity, taking the derivative of speed gives you the rate of change of the magnitude of velocity, which is acceleration.

In mathematical terms:

* Speed (v): v = |velocity|

* Acceleration (a): a = dv/dt (the derivative of speed with respect to time)

This means acceleration tells you how quickly an object's speed is changing. A positive acceleration means the speed is increasing, while a negative acceleration (sometimes called deceleration) means the speed is decreasing.