of是一个介词.英语里,介词后面必须跟一个名词.
有的单词既是介词又是副词.副词是可以放在句尾,什么都不跟的.
They made fun of her because she is overweight.
但是,有一种例外：
I hate being made fun of.(这是被动语态,所以,后面没东西.其实,这个介词的宾语被前移当作主语了)