A new model explaining this mysterious process suggests the core was created as dribs and drabs of iron percolated inward from Earth's lower mantle, according to a study published Oct. 6 in the journal Nature Geoscience. The mantle is the viscous, rocky layer between the crust and Earth's iron core.
Definition of flash card. : a card bearing words, numbers, or pictures that is briefly displayed (as by a teacher to a class) usually as a learning aid.
Quizlet was conceived by Andrew Sutherland in 2005 and has since made over 3 million sets of flashcards (that's a lot). The site accumulated over one million registered users in less than two years. I think the history of the application is very interesting.