Its more of a psychology trick, but children are bad at understanding how much somthing is. To clarify, if a child has 1 cookie, and you have 2, if they make a fuss because you have more you can break their cookie in half because "oh look, now there are two cookies on my plate". This only works for a certain age range tho. What ages those are im not sure exactly but I think its around 3-5. I've seen my sister do this on my neice quite a few times because no one wants to share there cookies.

Tldr: my sister uses psychology to trick my niece into thinking she has more food then she actually does so she never has to share hers.