Foliated metamorphic rocks don't break along their outer edges, they break along planes of weakness, which are parallel to the direction of pressure they experienced during metamorphism. These planes are what give foliated rocks their characteristic layered appearance.

Here's a breakdown:

* Foliation: The development of parallel layers or bands in a rock due to pressure during metamorphism.

* Planes of weakness: These are areas where the rock is most susceptible to breaking. In foliated rocks, these planes are created by the alignment of minerals under pressure.

Therefore, foliated rocks break along their foliation planes, not their outer edges. Think of it like splitting a deck of cards. You can easily separate the cards along their edges, which are the planes of weakness in this analogy.