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A snake’s slithering — how it translates wiggling motion into forward movement — has always been a bit of a mystery. Over the years, researchers developed the idea that an undulating snake drove its flanks laterally against small objects, like rocks and twigs, to propel itself.

“But that didn’t explain how snakes can move in areas where there isn’t anything to push on,” said David L. Hu, of Georgia Institute of Technology and New York University.

Now Dr. Hu and colleagues have come up with an alternative explanation, one that doesn’t rely on external objects. The secret, they report in The Proceedings of the National Academy of Sciences, is in the snake’s scales, which create different amounts of friction depending on direction.

The researchers did experiments with milk snakes and other species, and used the results to develop a model for slithering mechanics. They put snakes on extremely smooth surfaces, and in other instances wrapped them in cloth. In both cases, the snakes were unable to move forward, no matter how much they wriggled. (Videos are at nytimes.com/science.)

A snake’s scales, Dr. Hu said, resemble overlapping Venetian blinds, and tend to catch on tiny variations in the surface they lie on. This friction is greater in the forward direction than in sideways directions, as it is with wheels and ice skates. This frictional difference results in the snake’s moving forward as it undulates. (And the lack of movement is explained by the fact that placing a snake on a slippery surface or wrapping it in a cloth makes the friction the same in all directions.)