Here's why:

* Acceleration is the rate of change of velocity.

* Velocity has both speed and direction.

* So, acceleration means a change in either speed, direction, or both.

In the case of an object speeding up, its acceleration is in the same direction as its velocity. This results in an increase in its speed.

Important Note: While acceleration often means speeding up, it can also mean slowing down (deceleration) or changing direction.