### Instruction: What is cross-country skiing?
### Response: Cross-country (or "nordic") skiing, as opposed to downhill skiing, refer to the winter snow sport where the participant travels across relatively flat ground without the assistance of a chairlift or other machine aid in getting uphill. There are three main categories of cross-country ski available: classic, skate and touring, all of which come with specialized types of skies. Classic cross-country skiing involves a striding technique in which the skier keeps their skis parallel to each other and is best done in a narrow, groomed track. Skate skiing is considered more challenging to learn and involves a skiing technique that resembles the motion of ice skating and roller blading. Skate skiers also typically use a groomed trail but it is wider and smoother than the narrow grooves for classic skiing. Touring skis allows skiers to most easily ski off the trails on top of snow that has not been groomed.