To attract a piece of jade by laying a brick
During the Tang dynasty (A.D. 618-907) in China, in a city called Wu (today Suzhou city), there lived a poet named Chang Jian. Although Chang had gained a reputation as a gifted poet, he was dissatisfied with his own poetical works. In fact, Chang greatly admired Zhao Gu, another renowned oet, who he longed to meet for long, long time.
One day, CHang was so excited to learn that Zhao would be traveling to Wu. Knowing it would be once-in-a-blue-moon chance to exchange some poetic ideas with Zhao, Chang came up with lexx-awkward way to be acquainted with his hero.
Chang predicted that Zhao would possibly go sightseeing in Wu's most popular temple, Ling an Temple, during his trip, so he went there ahead of time. When he got there, Chang wrote a poem on the wall. However, instead of making the poem four lines as typical Chinese poems, Chang had only written two lines, leaving his poem incomplete.
'When Zhao comes here and sees this poem, I'm sure he will complete it.' Chang thought.
As Chang expected, Zhao visited the temple, and saw the unfinished poem and could not help adding another two lines to it.
This idiom literally means 'to lay a brick in order to attract a piece of jade.' As a brick is an inexpensive material and a piece of jade is a precious green stone, it implies that: to attract something valuable by using something valueless.