Flying squirrels do not actually fly, they glide using a patagium created by a fold of skin. From atop of trees, flying squirrels can initiate glides from a running start or from a stationary position by bringing their limbs under the body, retracting their heads, and then propelling themselves off the tree. It is believed that they use triangulation to estimate the distance of the landing as they often lean out and pivot from side to side before jumping. Once in the air, they form an "X" with their limbs, causing their membrane to stretch into a square-like shape and glide down at angles of 30 to 40 degrees. They maneuver with great efficiency in the air, making 90 degree turns around obstacles if needed. Just before reaching a tree, they raise their flattened tails which abruptly changes their trajectory upwards, and point all of their limbs forward to create a parachute effect with the membrane in order to reduce the shock of landing. The limbs absorb the remainder of the impact, and the squirrels immediately run to the other side of the trunk or to the top of the tree in order to avoid any potential predators. Although graceful in flight, they are very clumsy walkers and if they happen to be on the ground in the presence of danger, they will prefer to hide rather than attempt an escape.