The idea that Concepts and Categories are organized around prototypical examples and Family Resemblances.
For example, the concept of "bird" is anchored by small flying birds like sparrows and robins. Birds like penguins are both "technically" birds and also not as birdlike as the prototypes.
Penguins still count as birds because they have enough of the properties of birds (wings, beaks, lay eggs, etc.) to "fit" along with the prototypes.
An extreme example is the beanbag chair, which is very far from a prototypical chair. It squeaks by because, I suppose, it's *manufactured* for *sitting* (distinguishing it from a log, even a log that's had its top flattened) and maybe because when you sit on it, your weight creates vaguely arm-like shapes to your sides.
More expansively, Lakoff and Johnson believe Interactional Properties Are Key to Concepts.
