Classic condition was first discovered by Ivan Pavlov. The idea is simple: pair two stimuli together to create the same reaction. The most famous example of this is his experiment with dogs. He paired to stimuli, food and a bell. He would ring the bell and shortly after give the dog food. Eventually the dog would hear the bell and think this means I'm getting food and would cause the dog to salivate.
This made me think, what stimuli make me react falsely like the dog to the bell? Here's one that came to mind: picture yourself in the shower. When the water gets too hot you step out from under the shower. Seems simple right? Now you hear a toilet flush and if you're smart you'll step out of the water. This seems unnatural though. Why does a toilet flushing make me step out of the water? It's simple. Just like the dog learned that the bell meant food humans have learned that a flushing toilet means hotter water.
Next time you react, no matter how simple, I challenge you to consider and question why you reacted in the manner you did.
