How do snakes move forward and even climb trees, with no feet or hands? asks a reader.

Slithering snakes look like they're slip-sliding away. But snake locomotion actually depends on friction, just like walking. (Ever try strolling across a smooth patch of ice? It's a slippery reminder of how the friction between your feet and the sidewalk keeps you moving forward.)

Snakes move themselves along a surface mostly by squeezing and releasing muscles around their hundreds of ribs. These muscles are attached to skin covered with belly scales, which grip the ground like tire treads.

While snake species move in a variety of ways, all snakes can do the undulating crawl known as slithering. This familiar S-shaped motion is actually the speediest of snake movements, with the fastest-known snake gliding forward at about 12 mph.

Slithering snakes use their flanks to push off objects on either side -- like pebbles in a driveway. Snakes can even undulate through a pond, by pushing against the resistance of the water around them.

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But snakes also slither nicely across a piece of rough cloth. Scientists have known since the 1940s that snakeskin is good at generating friction with surfaces, helping a snake move steadily along. But it wasn't until the last few years that they figured out how a snake's hundreds of scales help propel it across your lawn -- and then up the nearest tree.

Scientists knew that the scales -- shaped like tiny clamshells, and overlapped like miniblinds -- helped a snake move along by catching on an uneven surface. In 2009, a study found that the friction between a snake's belly scales and a surface was highest when a snake was traveling forward, while sliding from side to side. By slithering and shifting its weight, allowing for more or less contact (and thus friction) with the ground, a snake could move quickly across a rough surface.

Then, in 2012, researchers at the Georgia Institute of Technology and New York University made a startling discovery: A snake can apparently control each of his belly scales, varying the scale's angle to the ground. By deliberately moving each tiny skin flap, researchers say, snakes can double the amount of resistance between scales and the surface they're moving across, substantially increasing grip strength.

The researchers also found that snakes on a bark-covered wall used their muscles to push on the bark with a force equal to nine times their body weight. Add angling their scales to catch on nooks and crannies, and snakes are able to slither up tree trunks -- rather like a climber using hands and feet to scale a vertical cliff.