The words mood, mooed sound the same but have different meanings and spellings. Why do mood, mooed sound the same even though they are completely different words?
The answer is simple: mood, mooed are homophones of the English language.
A state of mind or emotion.
A pervading impression of an observer: the somber mood of the painting.
An incidence of sulking or angry behavior.
Simple past tense and past participle of moo.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.