Yes, the words foolish, stupid and silly CAN be interchangeable. But, I would say foolish (and stupid) is more for those who make bad decisions. If your friend went out last night and went home with some random guy, you could say "She is so foolish (or stupid)- she should be more careful." but, at least in California, the word "foolish" isn't used very often, and we use the word "stupid" more. Silly is more of a happy word. If someone makes a lot of jokes, or doesn't take situations seriously, you could say "you're so silly." I would say children are always silly- they're the best way to describe "silly!" Intelligent and smart can also be interchangeable. By definition, intelligence is "the ability to acquire and apply knowledge and skills." and smart is "having or showing a quick-witted intelligence." They're basically the same word. I would say your intelligence is more of how you solve problems. In "smart", you could be street smart (know how to work the outside world and "streets"), book smart (comprehension and retention of learning by book, and classroom is high), or computer smart (good with computers), and so on.. Does that make sense?