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A ski is a long flat device designed to help the user slide over snow. A ski is pointed and curved upwards at the front to prevent it digging into the snow and the user is attached by a bindings to the skier's ski boots. Also, a ski may denote a similar device used for other purposes than skiing, e.g., for steering snowmobiles.
Snow skis glide on snow because downward pressure, as well as heat from surface friction, melts the snow directly under the ski. This creates a very thin layer of water directly under the ski upon which the ski glides. This is why if there is freezing rain that freezes to the bottom of the ski (perhaps when carrying the ski), when set down on the snow, won't glide until the ice wears off or is knocked off. Ski wax is used to increase the freezing point of water on the base of the ski, easing the creation of the water layer.
Skis were originally wooden planks made from a single piece of wood. They are now usually made from a complex assembly of components including glass fiber, Kevlar, Titanium or composite materials, though many may still contain a wooden core.
Most varieties of skis have a metal edge running the length of the sides that once sharpened allow the ski to grip more effectively on hard packed snow and ice.
The sides of all skis describe a parabola, making the ski narrower under the skier's foot than at the tip and tail. By setting the ski at an angle so that the edge cuts into the snow, the ski will follow the parabola and hence turn the skier, a practice known as carving a turn. Faced by competition from snowboarding, during the 1990s this shaping of the ski became significantly more pronounced to make it both easier for skiers to carve turns, and to dramatically increase the turning sensation experienced. Such skis were once termed carving skis, or shaped skis or parabolic skis to differentiate them from the more traditional straighter skis, but nearly all modern skis have this more pronounced shape now. For other turning techniques, see Skiing.
Many types of skis exist, all designed for use different situations, of which the following are a selection.
steep slopes furs can be attached at the base of the ski and the binding opened at the heel. For skiing downhill the bindings are locked. The ski is used with alpine touring boots which are hard but lighter than downhill skiing boots.