I don't quite understand classical conditioning, can you talk me through it?

Some stimuli provoke an innate reaction—for example, a loud noise causes most people to startle. If I suddenly made a loud noise and you weren't expecting it, you'd probably jump a little. There are some stimuli that don't provoke any innate reaction—like a pen. There's no natural or expected response to someone seeing a pen. That's because the pen is a neutral stimulus, whereas the loud noise is an unconditioned stimulus—and the startling is an unconditioned response to that unconditioned stimulus.However, if every time I picked up my pen I made a loud noise, you'd eventually start to associate the two together, and would startle or tense at seeing me pick up the pen, even if I didn't make the noise. That's classical conditioning. I've used an unconditioned stimulus (the noise) paired with a neutral stimulus (the pen) to produce a conditioned stimulus (the pen) and provoke a conditioned response (startling).