Here's why:

* Acceleration is a measure of how quickly an object's velocity changes.

* Velocity is measured in meters per second (m/s).

* Therefore, acceleration is measured in meters per second per second, which is written as m/s².

You might also see other units used, depending on the context:

* Feet per second squared (ft/s²): This is commonly used in the United States.

* Galileo (Gal): This is a smaller unit named after Galileo Galilei, who made significant contributions to our understanding of gravity. One Galileo is equal to 0.01 m/s².

However, the most common and preferred unit for free-fall acceleration is m/s².