 I'm going to give you a horrific example of a stimulus-stimulus pairing. I could present both of these stimuli to you at the same time, and hopefully eventually those two stimuli will take on the properties of each other. But how could the keys take on the property of the water? They can't. Anyway, you get the idea. The point is, is that we can do some fun stuff with stimulus-stimulus pairings, where we present two things concurrently, right, and over time, and with the appropriate procedures, those two stimuli can take on and share the effect of each other on behavior. Really what we're talking about gets back to stimulus control, among some other things, but it's about pairing two stimuli together to produce a particular effect and sharing the effect between the stimuli.