NO - Possibly the most used word by a parent.

Never say No? - An interesting post by Cee Kay on how to say "No" with a postive attitude to kids and why "No" should nt be over used.

Yes, we'll buy that toy some other time.
No, we can't buy that toy right
now.

Yes, you can have potato chips as soon as you finish this cucumber.
No
you cannot have potato chips. you have to finish this cucumber first.

Yes, we will go to the park as soon as you finish your dinner.
No, we
aren't going to the park until you finish your dinner.

Yes, I'll give you candy after you finish your lunch.
No, you cannot
have candy. Finish your lunch first!

And more recently -
Yes, we will think about getting you a laptop
when you are a little older.
No, you cannot get a laptop until you are a
little older.

And,
Yes, an iPod sounds fun, but we'll think about it.
You can't
have an iPod for Christmas. We'll have to think about it first.