〔R49〕  I am often seen on the ground;

                Flat and smooth I am often found.

                Only five letters make my name;

                Forward or backwards, I'm the same.

〔譯註〕 我常被看見在地上；

常被發現扁平而光滑；

只有五個字母造成我的名，

往前或往後看，我都是一個樣。

〔提問〕 If I am an instrument (器物) for determining (判定) a surface (表面), what am I?

〔謎底〕 l____