Here's why:

* Speed tells us how fast something is moving. It's a scalar quantity, meaning it only has magnitude (e.g., 50 miles per hour).

* Direction tells us which way something is moving. It's also a scalar quantity.

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction (e.g., 50 miles per hour north).

Example:

* A car traveling at 60 mph is only describing its speed.

* A car traveling at 60 mph north is describing its velocity.