Calvin and Hobbes by Bill Watterson for August 19, 1992
Calvin: Mom wants me to try an experiment tonight. She says the monsters under my bed may need me to think about them to exist. Her theory is that if I just don't think about them, they'll go away. Hobbes: ...Of course, that idea of being dragged under the bed and devoured by monsters has a way of gripping the mind. Calvin: And it's not like Mom and Dad go away when I stop thinking about them.