Another phenomenon that is common in reading Classical Chinese is homophony (words that sound the same). More than 2,500 years of sound change separates Classical Chinese from any modern variety, so when reading Classical Chinese in any modern variety of Chinese (especially Mandarin) or in Japanese, Korean, or Vietnamese, many characters which originally had different pronunciations have become homonyms. There is a famous Classical Chinese poem written in the early 20th century by the linguist Chao Yuen Ren called the "Lion-Eating Poet in the Stone Den", which contains only words that are now pronounced with various tones in Mandarin. It was written to show how Classical Chinese has become an impractical language for speakers of modern Chinese because Classical Chinese when spoken aloud is largely incomprehensible. However the poem is perfectly comprehensible when read silently because Literary Chinese, by its very nature as a "written" language using a logographic writing system, can often get away with using homophones that even in spoken Old Chinese would not have been distinguishable in any way.