Small, thin, plain, cards for writing your own messages, as in flash-cards

If you search for flash cards on the WWW you will probably find thousands of small memory devices for computers. Here we are selling small memory devices for human brains. The idea is to write simple questions on one side of the card, and the answer on the other. At a later stage you read all the questions and think of the answer before checking it out by turning the card over. The cards are sorted into two piles as you answer them - wrong ones, and correct ones. The wrong ones are put back into the "to do" bundle.