Pavlov's dogs were trained to expect food when a bell was rang. End result was that the dogs would salivate even if no food was present if they heard the bell ring.
Schroedinger's cat is a thought experiment about quantum physics. Schroedinger proposed that if a cat was placed into a box along with a device which had a random change of killing the cat after a random time, with no way to observe the result at any given moment without opening the box, that if you considered the box in that context, you had to conclude that the cat was both alive and dead at the same time until you opened the box. It has to do with frame of reference, quantum wave functions and the concept that quantum particles _can_ assume multiple states and that the states are fixed by observing the particles, and not an instant before (at least in your frame of reference, you can't know). TL:DR; schroedinger's cat is in an unknowable state.
So, to put that all together, it's funny because it rings a bell (related to pavlov's dogs) but whether the book was there or not was unknowable (like the state of schroedinger's cat).
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