Russian psychologist Lev Vygotsky proposed a "socio-cultural learning theory" that emphasized the impact of social and cultural experiences on individual thinking and the development of mental processes. Vygotsky's theory emerged in the 1930s and is still discussed today as a means of improving and reforming educational practices. In Vygotsky's theories of learning, he also postulated the theory of the zone of proximal development. This theory ties in with children building off prior knowledge and gaining new knowledge related to skills they already have. In the theory it describes how new knowledge or skills are taken in if they are not fully learned but are starting to emerge. A teacher or older friend lends support to a child learning a skill, be it building a block castle, tying a shoe, or writing one's name. As the child becomes more capable of the steps of the activity, the adult or older child withdraws supports gradually, until the child is competent completing the process on his/her own. This is done within that activity's zone—the distance between where the child is, and where he potentially will be. In each zone of proximal development, they build on skills and grow by learning more skills in their proximal development range. They build on the skills by being guided by teachers and parents. They must build from where they are in their zone of proximal development.