haha i like this question :)
speed is just one thing: the rate of movement on it's own.However, a velocity is a little more technical.
A velocity has two parts to it:
1. the rate of movement (the same as speed, above)
as well as;
2. the direction of motion.
SO you can imagine that as the tortoise goes around it keeps facing different directions so although for both speed and velocity the rate of movement is the same, the direction of motion is changing for the velocity.
This means that for circular motion, speed is constant but velocity is changing.
Now, any time you have a changing velocity (because of a change in its rate of motion or direction, or both) we say that there is acceleration.
So we say that although the tortoise has a constant speed, it is constantly accelerating due to it's circular motion.
