object lesson

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A concrete illustration of a moral or principle.
• noun A lesson taught by using a material object.

from The Century Dictionary.

• noun A lesson in which instruction is communicated, or a subject made clear, by presenting to the eye the object to be described, or a representation of it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a lesson taught (especially to young children) using a familiar or unusual object as a focus
• noun an example that typifies a principle
• noun a punishment intended as a deterrent to others
• noun idiomatic 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun punishment intended as a warning to others