Let me begin today's discussion of small-cap investment opportunities with a question that's at once whimsical and rhetorical: If you could invest in Superman, would you?

Allow me to introduce .
. American Science & Engineering (NASDAQ:ASEI), a little company out of Billerica, Mass., that can see through walls. And cargo containers. And clothing. Think of it as your way to invest in a "tracking stock" valuing Superman's X-ray vision.

This Motley Fool Rule Breakers selection develops and sells "nonintrusive inspection and screening devices" that the military, law-enforcement agencies, U.S. Customs, and Homeland Security all use to detect threats to the nation -- be they hidden in a shipborne cargo container, in a train-hitched railcar, or under the trenchcoat of that little guy with the shifty eyes who's waiting in line to board your international flight to Istanbul ahead of you. As we speak, the company's "Backscatter" X-ray technology is making headlines and giving the ACLU a hissy fit over invasion-of-privacy concerns -- all thanks to Backscatter's ability to see right through the clothing of an airline passenger, to detect not just metallic box-cutters but also ceramic firearms, plastic explosives, and contraband bottles of water larger than two fluid ounces.

What price safety?
So long as terrorism remains a concern in our post-9/11 world (i.e., for the rest of our natural lives), a company with technology like this is going to be in demand. But as investors -- even rule-breaking investors searching for the next big thing -- we owe it to ourselves to take a breath before plunking down our hard-earned cash, and read the price tag. So how much are shares of AS&E selling for these days, anyway?

AS&E earned $25 million for its shareholders over the past year; its free cash flow shows it to be nearly twice as profitable on a cash basis as it is under GAAP. Over the past four quarters, AS&E has generated a whopping $49 million in green for a price-to-free cash flow ratio of less than 13, a discount to its projected growth rate of 20%.

Over on the Fool's own rule-breaking stock-evaluation service, Motley Fool CAPS, nearly 500 investors have combined to make AS&E a five-star stock. It has 475 outperform ratings to just 14 underperforms, with a total of 225 of our all-star raters giving AS&E the thumbs up. Whether you want to correct their misconceptions -- or if you just want to shout "me, too!" -- you can rate AS&E, along with more than 2,000 other companies, on CAPS. Check it out for yourself to see what all of the fuss is about.

Seen our other contenders for best small cap? If not, click here.

Fool contributor Rich Smith has no position in any of the companies mentioned in this article. If he did, The Motley Fool would require him to tell you so. We're sticklers about things like that.