What is an object lesson?

[Takenrerei]

English Language Tests, Intermediate level

ESL/EFL Test #406 "English Slang Idioms (141)", question 9

"I need to bring this up as an object ......... to all of you. If you are rude to a customer, for any reason, you will get fired. I just fired Luke because he insulted a customer," the manager told the staff.

(a) teach
(b) manual
(c) lesson
(d) example

English Language Tests, Intermediate level

ESL/EFL Test #406 "English Slang Idioms (141)", answer 9

"I need to bring this up as an object lesson to all of you. If you are rude to a customer, for any reason, you will get fired. I just fired Luke because he insulted a customer," the manager told the staff.

Correct answer: (c) lesson
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Why is 'lesson' more appropriate than 'example'? Luke is an example of someone who is rude.

[Mister Micawber]

'Object lesson' is a fixed phrase; 'example' does not fit.
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Native English teacher at Mister Micawber's