It is very normal and expected for a toddler to say no and find it fun to say it. They like to use new words as much as possible to explore how it feels to say it and to see your reaction to how he uses it. You can try to create situations where he won't be able to use it, but is challenging and he may say it regardless of what you do because he doesn't have a full understanding of the purpose and effects of using the word. I've found that with every challenge with my kids such as this that the most effective way to deal with it is to continue to reinforce that it is only acceptable in certain situations and to keep reinforcing good manners. Also, make sure that your reaction to his behavior is as even as possible, without showing too much emotion around it. It's hard at 1.5 - he is still so young. But it will get easier and before you know it that won't be an issue and he'll be on to something else that is problematic :) Don't feel bad or give up if it takes a long time for this to pass. Try responding by having him say yes instead of no, just to get him to the point of using both words and as he grows older he will learn to differentiate for appropriate situations. When he says no you can correct him and when he says yes or some other answer that you wanted you should praise him a lot, using hugs, tickles, stickers, or whatever you think he'd enjoy. I've found that very helpful with my kids.