Across New York, a law-abiding person can walk into a sporting goods or hardware store and buy a standard, pocket-size folding knife — and be breaking the law.

While exact numbers are not available, thousands of New Yorkers are arrested every year and charged with possessing a “gravity knife,” which the law defines as one that opens with “the force of gravity or the application of centrifugal force.” In other words, with a flick of the wrist.

In 1958, state legislators banned knives that had a blade that fell out of the handle when the user pointed it at the ground and pushed a lever. The same law bans weapons like brass knuckles and “Kung Fu stars.” But the modern knives sold in countless stores bear little resemblance to the knives that were the original subjects of the ban. Many people, including carpenters, construction workers and stagehands, have no idea that their knives can be made to open with a flick of a wrist — a skill many New York police officers have developed. Most don’t know that simply possessing such a knife breaks the law.