In mental spaces theory, a relation can be assigned as holding between elements in a given mental space. In the following sentence: In the picture, a witch is riding a unicorn, the expression In the picture is a space-builder and sets up a mental space. The expressions a witch and a unicorn assign two elements to the mental space, while is riding prompts for a riding frame (or schema) to structure the mental space. This is achieved via schema induction. Once present, the roles associated with the riding frame are mapped onto the two elements, establishing a relation between them. While the witch element is associated with the rider role, the unicorn element is associated with the ridee role.