object lesson

a practical or concrete illustration of a principle.

Origin of object lesson

First recorded in 1825–35

An object-lesson in—might n't one call it the science of Applied Cynicism?

The Cardinal's Snuff-Box |Henry Harland
No object-lesson is more needed in the Celestial Empire than this.

In Kali's Country |Emily Churchill Thompson Sheets
It was an object-lesson for them, a clear illustration of what would happen to them if they ever disobeyed.

Astounding Stories of Super-Science, June, 1930 |Various
We had an object-lesson on the advantages of a reputation for being unamiable.

Fifty-One Years of Victorian Life |Margaret Elizabeth Leigh Child-Villiers, Countess of Jersey

But I still think the object-lesson was well worth a Napoleon.

Ruskin Relics |W. G. Collingwood

object lesson

a convincing demonstration of some principle or ideal

(esp formerly) a lesson in which a material object forms the basis of the teaching and is available to be inspected