"Fool" is a noun or a verb. You mean "foolish", ie. Fulano is a foolish person; Stop being foolish. Well, yes, there is a difference. "Silly" can mean comical and light-hearted. "Foolish" can mean stupid or gullible. http://www.thefreedictionary.com/foolish http://www.thefreedictionary.com/silly Even though there are a couple of times you could use one or the other (eg. foolish/silly = "idiotic"), I suggest you look at the differences, not the similarities.