〔R116〕 I have a pair of legs,

                 Which are pointed like pegs.

                  One of them never stirs nor jerks,

                  While the other moves and works,

                  That I may make a perfect shape,

                  Which you use to draw a grape.

〔譯註〕 我有腿一雙，

                 尖尖的像木樁。

                 一隻不動也不搖，

                 另一隻邊走邊辛勞，

                 好讓我把形狀弄完好，

                 以便你用來畫葡萄。

〔提問〕 If I am an instrument (器具) often used in a math class (數學課)，what am I?

 〔謎底〕  c______