Why did Americans change "s" to "z" in many english words?

cant think of examples now but u know what i mean?
Update: oh yeah i meanto add that too, taking the u out of colour and etc....
Update 2: amelia - i dont wanna doubt u - but u mean to say the irish accent was invented in the last 100 years to seperate us from the brits? we didnt av an english accent to begin with AND we have a totaly different language - do u mean idioms and slang and stuff? hardly accent....explain....! :)
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