A death mask thought to be that of English dramatist William Shakespeare (1566–1616). Found by Dr. Ludwig Becker in Mainz in 1849, the mask was linked to Shakespeare because of its 1616 date and its supposed facial resemblance to the writer. A rival theory, however, maintains that the mask is more likely to be that of English poet Ben Johnson. (Photo by Hulton Archive/Getty Images). Circa 1900

Ucil, a trained monkey, takes part in a street performance on June 1, 2011 in Jakarta, Indonesia. The street performances usually involve the monkeys wearing masks, such as dolls' heads or attire to mimic humans, with the monkeys trained to act out human activities such as shopping, riding bicycles or other simulations of human behaviour. Poverty drives the handlers to exploit the monkeys in the hope of earning small change, but the effect and cruelty to the monkeys is a cause that charities such as the Jakarta Animal Aid Network are increasingly taking up. (Photo by Ulet Ifansasti/Getty Images)

Some of London's telephone operators and engineers, trying out their gas masks at the anti-gas civilian training centre in Gloucester, where they were taught how to deal with gas attacks. (Photo by Harry Todd/Fox Photos/Getty Images). 13th February 1937

Three workers demonstrating the different types of gas masks used in mine rescue work. (Photo by Fox Photos/Getty Images). December 1925

A rare turquoise mosaic mask is displayed at The Britrish Museum on April 7, 2009 in London, England. The Aztec mask, originally from Mexico will be one of the key exhibits to go on show at the Moctezuma: Aztec Ruler exhibition which opens to the public on 24 September 2009. (Photo by Dan Kitwood/Getty Images)