or: the good part
A bit of Science - the "how does it really work" bit:
our brains build models of the external and internal world.
You could say that we have a virtual reality generator inside our heads that tries to anticipate the consequences of action (it's own and others).
Watch a puma or some other fast predator in the act of catching prey:
the puma's brain has to factor in speed (it's own and that of the prey), anticipate the course the prey is likely to take so it can plot an intercept course (this bit is like Star Trek) ;-), factor in the weight of the prey (!) - otherwise the moment the Puma catches it it is thrown off balance, open the jaws just so - not to wide or it takes to long to close them - nor too little or you catch nothing at all.
Pretty complex huh?!
If the brain of a predator had to start from scratch every time the hunt is on it wouldn't be able to catch anything at all.
Decisions have to be made instantaneously - speed is of the essence. The same goes for the prey by the way - without the jaw thing of course. ;-)
So how does a brain build a model of the world and the body?
Well - watch any toddler sitting at a desk - there comes a time during development when they start throwing things off the table - over and over again. They aren't called little scientists for nothing - throwing things is their way of learning about gravity. By throwing something off the table and hear it hit the floor beneath they learn that gravity is a constant and they are able to measure it's effect because the relation between the time it takes to hit the floor they can estimate the speed an object has.
There are literally hundreds of different experiments you can see going on during the first year of life - the visual tracking of objects for example. Infants - even when just a month old - learn that things in motion usually follow a steady course. That's why they are able to follow a point of light or an object on it's path even when the object is hidden from view for a short time. This truly is amazing stuff!
Again nature and nurture are at work here: the neuronal structures are laid down automatically - it's hardwired into our genome to develop these skills - but nurture "exercises" and strengthens these connections.
If you don't have the right environment present at the right time severe deficits turn up - as in the case of cats who couldn't move and were thus blind. Vision is dependent on movement - otherwise the brain can't make sense of the electrical signals coming in from the retina.
Not only does our brain contain models of the external world (physics, gravity, acceleration, ...) - but it also contains a body schema by having maps of the body surface all over the place.
The most important (and accessible) maps of the body are in an area called S1 - the Somatosensory Cortex - which is the funky way of saying "piece of brain that receives input from the outer body shell - skin".
And it is these maps that other parts of the brain use to run the virtual reality program - they are the basic building block of our sensory perceptions - or pre-perceptions as it were.
Say your brain wants to know how it would feel if you were to lift the arm straight up.
It doesn't send an actual motor command to do this - it runs an internal virtual reality simulation of you lifting the arm. It even anticipates the sensory feedback it would get from the receptors in your joints and muscles about you lifting your arm. These anticipatory sensory events are called pre-sensations.
The brain relies so much on these pre-sensations that it often acts on them instead on what really is happening. "Online" sensory feedback is only checked once in a while as it were. I will do a series on this some other time because it's such an important concept in treating chronic pain.
That way the brain can do every movement possible without you actually having to do anything in real life. This saves huge amounts of energy and time. Like in the example of the predator catching prey, saving time is a key function of our brains.
Again - nature and nurture: maps are hardwired - experience during childhood refine the maps (think babies sticking their feet into their mouths).
The big finding over the past decade or so has been that these maps are plastic and change constantly (within certain genetic boundaries).
If you use one part of your body more often the part of the map that corresponds is getting bigger - think violinists. One of their hand maps is huge.
If they stop playing the maps shrink again.
This has been shown for other forms of practiced movements too - juggling for example.
We have finally found one physical correlate of the motor skill learning process.
Even meditation - "just thinking" - is able to change your brain. Specific areas show remarkable differences between trained and untrained persons. This should convince even the greatest skeptics that the virtual reality simulation inside our brains is much more than a simple simulation - it actually is what the brain sees as being the real thing - a weakness that we can - among others - exploit in treating chronic pain.