Trail use has become very popular for a wide variety of users. Some trails are meant as nature trails, and are used by people learning about the natural world. Many trails are day trails, what means that they are generally used by people out for a short hike, less than a day. Some trails are backpacking trails, or long-distance trails, and are used by both day hikers and by backpackers. Some of the trails are over a thousand miles (1,500 km) long and may be hiked in sections by backpackers, or completed in one trip by dedicated hikers. Some trails are specifically used by other outdoor enthusiasts to gain access to another feature, such as good climbing sites. Many runners also favor running on trails rather than pavement, as giving a more vigorous work-out and better developing agility skills, as well as providing a more pleasant exercise environment. See trail running.