There are many things we see and hear, but aren't aware of. Awareness doesn't mean it doesn't affect us, though.
In the figure below, look at the pattern of brain activation when test subjects subliminally 'saw' words flashed up on the screen. The words were "masked" - viewed after a brief visual masking image, so that the subjects didn't know they had seen it. Because of the areas of brain that seemed to be activated, though it seemed that these subliminal words got filed on the basis of their meaning and imageability. It may be that this sort of subliminal information is an important reservoir of knowledge that we can use when we solve problems by "insight".
Interestingly, other researchers found subliminal learning was more likely to occur if test subjects were engaged in another task. These folks speculated that work on another activity activated a diffuse learning or reward signal that increased the likelihood that subliminal learning would take place.
If this is so, maybe it's good that some teachers act as a "sage on the stage" - rather than "guide on the side." If interest isn't captivated, maybe a whole lot less subliminal or incidental learning also goes on.
Unconscious Word Processing
Subliminal Learning Press release
Subliminal Learning Alters Nonconscious Memory Strategies