smoothing technique

例句：

1. 1. The surface of the metal is very smooth.
   金属的表面非常光滑。
2. 2. The road was smooth as glass.
   这条路像玻璃一样平滑。
3. 3. She smoothed down her hair with a comb.
   她用梳子把头发梳得顺滑。