Acyclic and cyclic alkanes are continuously in motion, flipping, twisting, bending and rotating externally and internally. At room temperature, ethane rotates about its sigma bond extremely fast. Each methyl group completing a full 360° rotation roughly every 10⁻¹¹ seconds. This rapid movement creates a variety of different spatial arrangements or conformations. If you could take a lot pictures of a molecule with a very very fast camera you would notice most of the pictures would correspond to conformations of low energy. Indeed molecules will spend most of their time in low energy stable conformations and much shorter amounts of time in high energy unstable conformations.