Go ahead. Get under the hood. If you don't wrench much, you may expect to find a mere collection of cranks and cams, a predictable combination of parts that can tell you exactly how high the engine will rev. Many of the so-called experts say the same about your portfolio -- that every fund or security you choose has set specifications, that rigid ratios and flashy formulas can drive your investment decisions. I used to think that was true, too, before I had much experience on my motorcycle or with my mutual funds. It took a serious breakdown outside Barstow on a BMW sportbike that had just received the shop's stamp of approval, and an equally grave failure by a growth fund blessed by all the authorities, to convince me otherwise. Investing -- or riding -- is no science. It's an art. Yet all too often, we make the mistake of assuming that because so much surrounding our fund is numeric, the buy-and-sell decisions are mathematical. Nothing could be further from the truth -- especially when considering one of the most difficult-to-understand (and measure) aspects of a mutual fund: its riskiness. Let's take a quick tour of some of the tools available to assess your mutual fund's risk (or its close cousin, volatility). We'll look at what's the best way to use them to better understand your investments, and perhaps more importantly, why none alone will give you a truly full picture of your portfolio.