I read this somewhere recently.

Problem:
The sentence uses the wrong relative pronoun.

Explanation:
The word “which” — spelled W-H-I-C-H — can be used as a relative pronoun to represent a specified antecedent.

Here are two examples of the correct use of “which” as as relative pronoun:

• In a nonrestrictive clause: “The movie, which I saw last night, was boring.”
• In a restrictive clause: “The religion which John preached was unacceptable to many.”

The problem with “which” as a relative pronoun in “They are the people which pay the bill.” is that “which” is supposed to refer to inanimate objects and to animals but not to humans.

The correct relative pronoun for referring to humans is “who”. This gives us the solution.

Solution:
“They are the people who pay the bill.”