* Acceleration is the rate of change of velocity. Velocity is speed *and* direction.

* Constant speed means the magnitude of your velocity (how fast you're going) is unchanging.

Think of it like this:

* Speeding up: You're accelerating, increasing your speed.

* Slowing down: You're also accelerating, but in the opposite direction of your motion (decelerating).

* Turning: Even if you maintain a constant speed, you're changing your direction, which means you're accelerating.

In short: To accelerate, you must either change your speed or your direction. Constant speed means you're not changing your speed, and therefore, you cannot be accelerating.