What is known as prior knowledge or pre-existing knowledge is the knowledge, skill or ability that a learner brings to a new learning encounter. This includes all knowledge that is available before the learning event, and which has been gathered or developed by any means, and in any situation, including both formal and, quite often, informal learning situations. Learners need enough previous knowledge and understanding to enable them to learn new things; they also need help making links with new and previous explicit knowledge. It is considered to be valuable to go through a process of what has been called activating prior knowledge. Teachers often go through this process at the beginning of a new topic. They also use introductory strategies at the beginning of lessons which are continuations from previous lessons. In terms of the practicalities of teaching, this is a process of making children think about the topic or remember what has been covered already. In terms of theory, it is to do with activating particular schemas.