Parenting.

Here is a brief example of a day in the life of a parent.
3 Year Old Boy: "Can I have a snack?"
Dad: "If you're still hungry you can have some more supper, or an apple or some broccoli or carrots, but that's all."
Boy: "Why?"
Dad: "Because you just had dessert. ."
Boy: "Yeah but that was dessert. Can I have a snack now?"
Dad: "No."
Boy: "But it's bedtime, I always have a bedtime snack."
Dad: "Yes, you had a banana for a bedtime snack."
Boy: "No that was dessert. I still need a bedtime snack."
Dad: "No."
Boy: "Dad?"
Dad: "Yes?"
Boy: "What can I have for a bedtime snack?"
Dad; "If you're still hungry you can have some more supper, or an apple or some broccoli or carrots."
Boy: "Dad, those aren't snacks."
Dad: "Son, all food outside of meal time is a snack."
Boy: (pause) "Dad, do we have any ice cream?"
Dad: "No."
Boy: "Can I have a cookie?"
Dad: "No. If you're still hungry you can have some more supper, or an apple or some broccoli or carrots, nothing else."
Boy: (voice rising with excitement and enthusiasm) "DAD! We still have Lucky Charms don't we?! I could have some Lucky Charms!"
Dad: "No. Those are for breakfast."
Boy: "I want some breakfast now."
Dad: "No. It's bedtime now, not breakfast. If you're still hungry you can have some more supper, or an apple or some broccoli or carrots, nothing else."
Boy: "Dad..."
Dad: "Son, I've told you what you can have."
Boy: "But I want a snack." (Go back to beginning, repeat 3 times, until son decides he'll have an apple.)