显微镜下，针尖上的小点看起来像一个小球。

Under the microscope, the tiny dot on the ___pinhead looks like a small ball.

别跟那个傻瓜争论了，他连简单的问题都搞不懂。

Don't argue with that ___pinhead, he can't even understand simple questions.

这个错误小得像针尖一样，几乎可以忽略不计。

The mistake is as tiny as a ___pinhead and almost negligible.