Fast

It occurred to me recently that the word "fast" can be a noun (e.g., breaking one's fast), a verb (to fast for three days), an adjective (a fast car) and an adverb (to drive fast). Granted, the noun/verb "fast" and the adjective/adverb "fast" are homonyms -- like "bear" (n.) and "bear" (v.), etc. -- rather than forms of the same word...but it's still pretty neat.