Here's why:

* Scalar quantities have magnitude (size) only. They don't have direction. Examples include:

* Temperature

* Mass

* Time

* Speed

* Vector quantities have both magnitude and direction. Examples include:

* Velocity

* Displacement

* Force

Speed tells you how fast something is moving, but it doesn't tell you the direction. Velocity, on the other hand, tells you both how fast something is moving and in what direction.