[quote name='Vinny']Because alcohol vaporizes very quickly. That's why the purest form of alcohol (a 99% alcohol solution, not sure if 100% exists) is best for cleaning carts, since 99% of that solution (in other words, the alcohol) will dry up within minutes. The moisture from your breath on the other hand stays a while, though you can't see it. It's kinda like humidity, except humidity normally doesn't go inside those carts unless it's forced in there somehow.
Plus, the alcohol only touches the chip board via a q-tip, not the actual circuits and chips and other crap inside those carts. In theory, I guess you could use your saliva to clean the connectors of the cart.

k I wouldn't try it though...
As a note, 99% isn't necessary. I used 70% without any trouble, and heard that anything near 90% will work well.[/quote]
They do have 100%, but it's worthless, it won't do anything.
Since it's 100% the alcohol can't react with water, so it just sits there, doing nothing.
And why you would want to clean out your NES games with Saliva is beyond me.