object lesson

Short definition from Wiktionary

Noun

1. 1
   A lesson taught (especially to young children) using a familiar or unusual object as a focus.
2. 2
   An example from real life that explains a principle or teaches a lesson.
3. 3
   Anything used as an example or lesson which serves to warn others as to the outcomes that result from a particular action or behavior, as exemplified by the fates of those who followed that course.