If someone set a marshmallow in front of you, would you eat it?
If that person told you that if you didn’t eat it, but watched it for 15 minutes, they’d give you another marshmallow. Would you eat it or wait?
Now imagine you’re a kid. Do you think you’d have the patience to wait 15 minutes? Think of how much longer 15 minutes seems like to a child than it does to us.