Here's why:

* Fundamental units: These are the basic building blocks of measurement. Examples include meter (m) for length, kilogram (kg) for mass, and second (s) for time.

* Derived units: These are units that are defined by combinations of fundamental units. They are "derived" from the fundamental units.

Examples of derived units:

* Speed: Meters per second (m/s) - derived from length (m) and time (s)

* Area: Square meters (m²) - derived from length (m) multiplied by itself

* Volume: Cubic meters (m³) - derived from length (m) multiplied by itself three times

* Density: Kilograms per cubic meter (kg/m³) - derived from mass (kg) and volume (m³)

* Force: Newtons (N) - derived from mass (kg), length (m), and time (s)

Essentially, derived units represent more complex physical quantities that can be expressed using the basic building blocks of fundamental units.