Know no

How does a 10 month old baby know what this means? It's sort of incredible if you think about it. You say "no"and they turn around, look at you, and then keep crawling but even faster.
They don't know words. They can't talk. Yet they understand basic English commands. They just choose to ignore your commands. You see, I believe this is where they start the bad habit. By the time they're two and they know what you're saying, it's too late. They are so used to getting away with "not knowing no" even though they know, no. Do you see what I'm saying?