The term is also used for things that remove marks from chalkboards and whiteboards. Slate was shaped into sinks, bathtubs, tabletops, chalkboards, roof shingles, and headstones. By the late 1990s, about 21% of American classrooms had converted from chalkboards to whiteboards. Elementary school students came to class with chalkboards, chalk and little else. A mischievous dog, Reveille V often stole erasers from chalkboards during class. Automobile squeegees are used in some universities to clean chalkboards. Other uses include writing on mirrors, chalkboards, plastics, ceramics, glass windows and other non-porous surfaces. The use of chalkboards, moveable walls, and other objects form barriers between learning areas. In this animation made by Martin using felt and chalkboards, Martin thinks about porcupines. Chalkboards make an ideal medium because it alludes to teaching and learning. On the final day, the inmates tried to escape using chalkboards and hostages as shields. She had them draw on chalkboards and squeeze the bulb of the dynamometer. Until recently, in some countries such chalkboards continued in use. He would stand before an audience, with a large number of chalkboards.