Here's why:
* Agile encompasses a broader sense of movement and adaptability, often with a focus on grace and efficiency.
* Nimble emphasizes dexterity and quickness, particularly with the hands.
* Alert refers to mental sharpness and attentiveness.
* Quick is a general term for speed.
* Brisk suggests a lively and energetic pace.
* Active indicates engagement and movement, but not necessarily with the same emphasis on speed or grace as agile.
While all these words are related, "agile" is the most encompassing and captures the essence of the group.