It's kind of like crop rotation (if you have the diversity of mind to accept a slightly off-center analogy). In farming, they say if we keep planting the same crops year in and year out, we greatly diminish the land's productive capacity, essentially stripping the soil of its nutrients. Whereas, when we do the same mental activity year in and year out, we hinder our mental productive capacity. Mental muscle atrophy, if you will.

Hurry up and hand me that crossword puzzle.