This seems to be the proper season to write about expressions that involve cakes, cookies, and other sweet stuff. So here is an interesting expression for you: “you can’t eat your cake and have it too”, it also works the other way around “you can’t have your cake and eat it too”. Both variations work although the first one (eat-have) seems the more traditional one and the second one (have-eat) the one more frequently used now.
The meaning is pretty straightforward: if you eat your cake, you don’t have it anymore. Eating and having it are mutually exclusive. Hence this proverb is used to express the impossibility of having something both ways, if those two ways conflict.
Therefore the expression is used mostly in the negative “you can’t have your cake and eat it too”.
An example for the usage of this expression is here:
Janet: “I really want those cute Manolo Blahnik shoes. But then I need to save money to afford to go to college.”
Jennifer: “Tough choice. But you can’t eat your cake and have it too.”