This little wooden frame contains the word 'FILM' in capital letters. However part of the word is on a sliding section, that can move to the other end of the frame. When you turn the frame the other way up, causing the moving section to slide to the other end, the word 'FILM' still appears. However it has now changed from black letters on a white background to white letters on a black background. A neat trick, almost an optical illusion!
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