When you look at it, you can see it in two different ways. You
can mentally shift from seeing it with one face as the front to
seeing that same face as the back of the cube.
There are other ``optical illusions'' which are similar. You can
see the picture as the profiles of two lovers or a single wine
goblet. You can see the old hag or the demure debutant.
Calling these ``optical'' really does them ill. When you're looking
at a Necker Cube, you can feel a mental shift. You can even
do it just imagining the cube in your mind. It has nothing to do
with your eyes or the light or anything.
You can make the same sort of shift when you're listening to
music. Listen to the song. Listen to it as a whole. Let all of
the instruments hit you at once. Now, pick an instrument and
follow it. Listen to that instrument. Now, pick a different
instrument (or the vocalist). You can shift around to various
individual elements or shift back to the whole thing. With a
little practice, you can shift to a couple of instruments... or
all of the instruments except one. Try it.
Now, try this one. You're probably using a web browser right
now. Take it to some page where you've got lots of room to
scroll up and down. This page may suit the purpose. Now,
use the scroll bar to move up and down the page. It makes
perfect sense. It's an entirely natural human-computer
interaction. You've done it a thousand times.
Now, do it again. Grab the scrollbar with your mouse. But,
this time, look for a moment at how the scrollbar compares
to the text on the screen. When you slide the scrollbar up,
the text slides down. When you slide the scrollbar down, the
text slides up. You've got the text on a pulley system.
Now, try to shift back and make it seem natural again. Wheee...