In contrast to other organisms, animals are able to move. The two forces an animal overcomes to move are gravity and friction. Aquatic animals do not have much difficulty overcoming gravity, since they are buoyant in water. However, because water is dense, the problem of resistance (friction) is greater for these animals. Many of them have sleek shapes to help them swim. Terrestrial animals tend to have fewer problems with friction since air has less resistance than water. However, terrestrial animals must work harder to overcome gravity.