Mountain bikes are typically ridden on mountain trails, fire roads, logging roads, and other unpaved environments. These types of terrain commonly include rocks, washouts, ruts, loose sand, loose gravel, roots, and steep grades (both inclines and declines). Mountain bikes are built to handle this terrain and the obstacles that are found in it like logs, vertical drop offs, and smaller boulders. There are two type of mountain bikes: Hardtail, which do not have a rear suspension; and full-suspension ones, which have front and rear suspensions.