For those of you who care, you use "fewer" when the number of items in question can be counted or quantified, and "less" when you are referring to a volume, amount, or other entity of measurement that cannot easily be enumerated.

It's literally the difference between "continuous" and "discrete" except in other words. LITERALLY. Use "fewer" for discrete entities and "less" for continuous. Water (on a normal scale) is continuous, if we're not going down to the atomic level (and we're not). Bottles of water are discrete.