If there is no good reason to say "no" to a child (or an adult for that matter), then say "yes." If there is a good reason to say "no," then say "no" clearly, kindly, and firmly.

Children and others need to know that there are limits--that instant gratification of our wants and even our needs is not always going to be possible or even desirable.  We as individuals, and in communities with others, will not benefit from the excesses of action or inaction to which we might be inclined due to either biological or learned influences.

Good reasons for saying "no" include situations in which the behavior is likely to be hurtful directly or indirectly through the inappropriate use of scarce resources.