Active recall is a principle of efficient learning, which claims the need to actively stimulate memory during the learning process. It contrasts with passive review, in which the learning material is processed passively (e.g. by reading, watching, etc.). For example, reading a text about George Washington, with no further action, is a passive review. Answering the question "Who was the first US President?", is active recall. There are several variations of active recall as a strategy and these include the 3R (read-recite-review) method by McDaniel et al. and Feynman technique.