C is for Cheeks

Cheeks of animals, for example of the pig, because they usually yield rich, savory juices, are a good choice to include in stews, pies, and sausages. However, because cheek muscles are exercised constantly, the meat tends to be tough and may need long cooking. Cod cheeks, on the other hand, are tender morsels, perhaps because the cod are not eating all the time and do not exercise their cheeks in making noises.

The Penguin Companion to Food

I must say I never thought of comparing pig's and cod's cheeks ... or that cheeks would tend to be tough.