Here's why:
* Adverbs modify verbs, adjectives, and other adverbs.
* Adverbs answer questions like: how, when, where, to what extent, and why.
Example:
* Verb: The dog ran quickly. (The adverb "quickly" describes how the dog ran.)
* Adjective: The dog was very happy. (The adverb "very" describes the degree of happiness.)
* Adverb: He ran extremely quickly. (The adverb "extremely" describes the degree of quickness.)
So, to describe an adverb, you use another adverb that modifies it!