I had Harley's tank on the floor for a while this summer, and one time my turtle went right up to the edge and tried to bite her head off through the glass (she's friendly that way). Harley didn't even move. Just stared her down. I think they know that there's something there, but they don't understand it. Like, Harley realizes that nothing can get to her while she's in her viv, but I don't think she understands the glass is the reason why. She always acts really confused that she can't smell anything through it.