To illustrate courage, one might say:
زَيْدٌ كَالْأَسَدِ فِي الشَّجَاعَةِ — Zayd is like a lion in courage.
The sentence does not identify Zayd as an actual lion. It compares Zayd with a lion in respect of bravery. The particle كَـ marks the comparison, while فِي الشَّجَاعَةِ states the shared quality.
Simile can illuminate an idea, not merely decorate its wording. This is an explanatory example, not an attributed quotation from the textbook.