I refer to the alleged truth that one must change one's routine regularly in order to ensure progress. What's being tricked here are those who believe such nonsense. Muscles don't think and reason and respond, like Pavlov's dog, to varied stimuli.
Muscles expand and contract in response to nerve impulses.
They perform work. You can't confuse, fool, trick, cajole, persuade, reason, entice, or otherwise communicate with your muscles. Sure, something has to change in order for the muscles to get bigger and stronger. So if you're training properly and adding weight to the bar regularly while maintaining proper form, you're making the one and only change required to stimulate an increase of protein synthesis within the myofibrils, which accounts for the vast majority of muscle size. The gaining they notice is in motor learning skills, not actual strength or contractile tissue gain.