"lack"幾時有"of"?

有好多人對於 lack 同lack of 會攪唔清, 而覺得後面有個 noun 前面加of 就無錯啦. 但英文係要變通, 唔係死記爛背架 !

lack (v.)
She lacks sympathy.

lacking (adj.)
There is something lacking in our classroom but I can't tell what it is.

lack (n.) of
The lack of rain in this town is going to cause a famine.