When Nike asked runners to envision their ideal sneaker, the answer came as a surprise — a sock? Four years and hundreds of prototypes later, Nike introduces the Flyknit, which the company claims delivers all the benefits of a (lightweight, foot-conforming, comfortable) sock plus advanced Nike engineering. (See the video below.) Here's what you should know:

What is the Flyknit made of?
The upper half is a web of yarn and flexible cables (to add support), while the lower half is a cushioned sole. The yarn is made of "feather-light, high-quality polyester," with "varying elasticity, durability, thickness, and strength," writes Anthonia Akitunde at Fast Company.

Is it that much lighter than other sneakers?
The Flyknit weighs just 5.6 ounces, which is 20 percent lighter than Nike's Zoom Streak 3, the shoe worn by the top three finishers in the men's marathon at the 2011 World Championships. If that doesn't impress you, consider this: Nike officials say that the difference, when "multiplied by the 40,000 steps it takes to run a marathon," equals "the weight of a car," says Adam Aston at Greenbiz.

When does the Flyknit make its debut?
Runners from Britain, Kenya, Russia, and the U.S. will don the shoe for the 2012 London Olympics, which begin July 27. Ordinary Joes can also pick up a pair come July. "No price announced yet, but assume expensive: Engineered yarn doesn't come cheap," says Dan Seitz at Guyism.

Sources: Fast Company, GlobalGrindGreenbiz, Guyism