So close
The glass barrier simply frustrates her attempts to get to the milk. She can see the glass, yet why can’t she perceive the barrier? Cats rushing headfirst into windows thinking they aren’t there are examples of cats that don’t understand glass as a barrier.
Cats are very active and clever; they will go to any length to get what they want, especially if it’s within their grasp. Unfortunately, though, as clear glass bends light in such a way that it looks like something is there when nothing is, they can see through it. Their eyes aren’t wired the same way ours are to be able to sense light refractions that help us tell glass from a barrier. As a result, cats are unable to detect fine reflections that we notice when looking at the window.