Here's a puzzle: Q: Why does America have 3 million empty jobs that employers are struggling to fill -- while millions remain unemployed in the wake of the Great Recession? A: Mostly because jobless people lack special skills needed for the vacant positions.

Columnist Thomas Friedman described a Minnesota welding shop that sought to hire 10 high-quality, nationally certified welders for a government contract to affix armor onto military Humvees.

Plenty of applicants wanted the $20-an-hour work -- but they had learned to weld in neighborhood garages or high school training classes and lacked top-grade expertise. Shop owner Traci Tapani explained:

"They did not know the science behind welding.... They could make beautiful welds, but they did not understand metallurgy, modern cleaning and brushing techniques.... You have to have science and math. I can't think of any job in my sheet metal fabricating company where math is not important. If you work in a manufacturing facility, you use math every day: You need to compute angles and understand what happens to a piece of metal when it's bent to a certain angle."

 Tapani finally trained a local female welder intensely enough to pass a national certified welding inspector test -- and the woman subsequently trained other welders for the Minnesota shop.

The moral of this tale is that many 21st century U.S. jobs require well-honed skills and credentials, often including STEM (science, technology, engineering and mathematics). Millions of young Americans aren't prepared for the new era. Their test scores lag behind.

Last year, a survey of 65 nations ranked U.S. students 23rd, equal with Poland. Worse, West Virginia trails America. The national Institute of Physics lists the Mountain state as 49th in "science and engineering readiness." One-fourth of teens drop out of high school, and many wreck themselves with dope -- so they have little chance for today's careers.

During the summer, two industrial experts wrote: "Today's manufacturers often rely on precision machinery, computer modeling and high-tech tooling far removed from the traditional assembly line, and too few American students are prepared for these skilled, internationally competitive jobs."

Three million unfilled jobs are waiting across America. But too many people can't be hired, because they lack proper knowledge. Every U.S. school system and most American families should focus on this learning gap, to ensure better opportunities for the young.