Lesson
A lesson in school is a planned period of time for learning. It involves one or more students being taught by a teacher. A lesson may be either one section of a textbook or a short period of time during which learners are taught about a subject or taught how to perform an activity. Lessons are generally taught in a classroom but may instead take place in a situated learning environment.
Also, a lesson also means a learner learning something he did not know before. Such a lesson can be either planned or accidental, enjoyable or painful. The slang phrase "to teach someone a lesson", means to punish or scold a person for a mistake they have made making sure that he does not make the same mistake again.
Lessons can also be made entertaining. When the term education is combined with entertainment, it is called "edutainment".
Lesson Media
"The Difficult Lesson" by William-Adolphe Bouguereau (1884)