Lately, I have gotten very frustrated by my children misunderstanding the word "No".
They will ask me for something (can I have a marshmallow for breakfast? can I stay up late and watch Clone Wars? I can put this book in the microwave? I get a facebook page?), I will clearly respond "No." They will ask again, repeatedly, and I will calmly answer "No" each time. An hour later, they will ask again.
I think "No" is a pretty clear message. Once in awhile I will give reasoning or an alternative to the request.
No means no.