Educators use teaching aids to impart and emphasize information, stimulate interest, and facilitate the learning process. They range from simple to sophisticated and can be aural, visual, or increasingly more frequently, computerized. One of the earliest known teaching aids is the hornbook, which was used in English schools from the mid-1400s (and later in colonial America) through the early nineteenth century. It was a flat board on which a sheet of paper printed with the alphabet, the Lord's prayer, and several simple words were pasted. A transparent piece of horn covered the paper to protect it. The blackboard probably evolved from the hornbook, and has traditionally been one of the most widely-used teaching aids. It was patented in 1823 by Samuel Read Hall of Concord, Vermont, a Congregational minister who founded the Concord Academy to train teachers in "school keeping." Hall's version was made of pine...