nub

例句：

1. 1. The scientist found a nub of gold in the rubble.
   科学家在废墟中发现了一块金块。
2. 2. The ball was so heavy that it rolled down the hill.
   这个球太重了，滚下了山坡。
3. 3. The sphere was perfectly round and smooth.
   这个球是完美的圆形和光滑的。