Once upon a time, there was a black and white panda named Patches who lived in a zoo. Patches was known for two things: his love of naps and his complete inability to tell the difference between a rock and a snack. 
One afternoon, a zookeeper gave him a big, juicy bamboo stalk, but Patches was too sleepy to notice. Instead, he picked up a nearby rock and started chewing on it, thinking it was just an oddly crunchy piece of bamboo. The zookeeper watched, confused, as Patches chewed away, blissfully unaware. 
After a few minutes, the panda paused, spat the rock out, and looked at it with a puzzled expression. “Well, that was weird,” he thought. Then, he casually rolled over, grabbed the bamboo, and went back to sleep, as if nothing had happened.
From that day on, the zookeepers made sure to give him only bamboo... unless they were trying to trick him for a good laugh.