The boys have a snack at 3:00 everyday. It's predictable, it's about halfway between lunch and dinner, it works well with our activities. It keeps them from asking for food all day long.
M (at 2:40): "Can we have snack?"
Mommy: "In twenty minutes."
M: "Oh. How about 3 minutes..."
Mommy (interrupting him): "No, at 3:00"
M: "...uh, 2 minutes, because then it would be zero minutes faster and I'm hungry and I don't like twenty minutes and I don't want to wait and it's not fair."
I had to laugh at his reasoning. Being four must be fun. He actually thought that might persuade me.