"No." Two letters make a big word. It is a word that indicates refusal, disinterest, or a different opinion.
Small children learn the word quickly and use it at every opportunity to assert their independence. It is a word parents use to protect a crawling child from danger.
It can clarify a statement to determine if it was dissent or confirmation
In my class, I have a student who says "no" every time an adult speaks to him. For him, "no" means..."Give me a minute to think." or "I don't know what you are saying, could you say it another way?" or "Help me know if saying yes is safe." Dor this young man, no isn't a refusal but rather a word he uses to protect himself from the unknown. His limited receptive language has caused him to agree to things he was not very happy about so he is cautious and anxious about agreeing until he understands. His limited expressive language and articulation struggles leave him with the simple but eloquent, no. Given time to process, he is usually happy to comply and grins with the success of both protecting himself from the unknown and pleasing the adult.
This young man reminds me every time he declares "no" that processing and think time is not only critical, it is the only way to give the student the feeling of safety and security that we all want for each of our students and children.