People crowd the corridor at Longhua Temple trying to solve the lantern riddles prior to the Lantern Festival, which falls on Sunday.(Photo Source: Shanghai Daily)

The Lantern Festival, which falls on Sunday this year, is a time to appreciate the night sky filled with lanterns, and especially to appreciate the wide-eyed children captivated by the lights.

But there's another ancient aspect to the festival that concludes the Chinese Lunar New Year celebration - the "tiger lantern" - also known as riddle guessing.

Because ancient riddles were so difficult and required so much classical knowledge, wisdom and wit, there was a saying about them: It's as difficult to kill a tiger as it is to guess a lantern puzzle. Hence, the name tiger lantern.

These days, however, most popular puzzles don't require a literati to come up with the answer, though they can be brain teasers, requiring knowledge of history, culture and classics, puns and homophones.

Today's Spring Festival puzzles usually carry simpler messages of good fortune, family reunion, abundant harvest, prosperity and love.

Lanterns often have four riddles written on red vertical strips of paper pasted to the lamp, attached by strings.

Winners these days usually get small gifts. In the past, the prize was intellectual satisfaction.

The guessing tradition goes back more than 2,000 years, when imperial advisors conveyed advice and opinion in metaphorical and oblique language, in an indirect way (speaking in riddles). The idea was to protect themselves from imperial wrath for delivering bad news and criticism - and of course to save the emperor's face, if his behavior and policy were at fault.

In the Tang Dynasty (AD 618-907), guessing riddles gradually became a contest of wit among men of letters to challenge and learn from each other. Now those were real tiger lanterns.

The game reached its peak in the Song Dynasty (960-1279) when it evolved into an essential part of the Lantern Festival for people from all walks of life.

Some people would cut out poems and verses and paste them onto silk lanterns to satirize or ridicule current events and people. Some were grotesque or humorous pictures of people, with hidden meanings and obscure words. They also posted current jokes - all intended to amuse passersby.

Traditional songs, poems, stories or historical events were often the subjects of the riddles.

"It's a game that needs empathy between the riddle guesser and the riddle maker," says Xu Hangming, director of the Shanghai Workers' Lantern Riddle Association.

The 40-year-old teacher from Tongji University has been engrossed in riddle study, guessing and creating for 25 years.

In 1998, Xu offered an optional course in lantern riddles that was quite popular among college students.

"You have to go along the right track of the riddle maker's thought," he says. "The greatest appeal of the game is not simply hitting the right answer, but following the riddle maker's thought processes and obscure clues, just like a hunter. You should know your opponent and yourself well."

Each riddle must have an answer.

"A riddle that no one can answer is a total failure," say Xu.

There are about 20 methods to solve different lantern riddles, based on the variation of the pronunciation, shape and meaning of Chinese characters. The better ones require knowledge of history and culture.

The key could be a single word, a line from a poem, a historical event, a place name or object.

The basic rule about a lantern riddle is that a word in the riddle cannot appear again in the answer, but there should be a sort of relation.