Well, yes, it is deep, as in possessing depth. That is another law of physics, probably law number 3, if that’s not already taken. In your mouth, how does one measure depth versus, say width, or height (oh, that’s another great term to abuse, I’m adding it to my list immediately)?
When I go through the motion of drinking, my head can bend forward then backward, then forward, and all my cardinal directions are getting screwed up. So, if I say depth, I’m saying depth like the 10-foot shine on a car. It’s the impression, the illusion of depth. It’s all, or almost all, an illusion anyway. Illusion of course is a synonym for BS -- thanks, Penn & Teller!