Yogi Berra once said, "You can observe a lot just by watching." Or maybe it was "You can hear a lot just by listening." I don't remember precisely, because when Yogi's quip came out of my clock radio, I was madly looking for my car keys (which were in my hand the whole time).
So what makes Yogi's aphorism—whatever it was—so maddeningly true? How can we look directly at things and not see them? The answer is that your brain perceives the world through what amounts to a mental "soda straw." When it aims that straw at one thing, all other objects—even those within your direct field of vision—recede into the background.
Your brain calls up remembered images through the same conduit it uses to take them in live. Close your eyes and visualize everything inside your living room, including the furniture, walls, and artwork. As you scan your memory, notice that even though your brain stores the contents of your living room in surprising detail, it only "sees" these objects one at a time, more or less. You may be able to "zoom out" to recall several parts of your living room at once, but when you do, the details of individual objects will become a mental blur. This blurring effect illustrates an important property of selective attention: Your brain not only discards information that's outside the soda straw but is able to slurp up only a limited amount of detail within the soda straw as well.
To make matters worse, inputs from all your sense organs must flow through the same narrow straw and may block each other. For instance, talking on a cell phone while driving "blinds" you to important details about traffic. This may explain why dipping your mental soda straw into an active phone conversation while driving is equivalent to dipping it into a stiff shot of whiskey.