The principle of solid drawing means taking into account forms in three-dimensional space, or giving them volume and weight.[12] The animator needs to be a skilled artist and has to understand the basics of three-dimensional shapes, anatomy, weight, balance, light and shadow, etc.[32] For the classical animator, this involved taking art classes and doing sketches from life.[33] One thing in particular that Johnston and Thomas warned against was creating "twins": characters whose left and right sides mirrored each other, and looked lifeless.[34] Modern-day computer animators draw less because of the facilities computers give them,[35] yet their work benefits greatly from a basic understanding of animation principles, and their additions to basic computer animation.[33]