Something visually boring - to the point of your eyes losing focus - that numbs your brain.
A sermon from dad elicits the "in one ear and out the other" response while a friend's boring home movie gets an "in one eye and out the other" reaction.
The stench was pretty brutal in stall 3 when the snake wizzard was in one hole and out the other. He sucked me dry and dumped his stinkyload simultaneously. Febreeze anyone?!