To find the height of a brick-kiln chimney, Anjali lays a small mirror flat on the ground 18 m from its base and steps back until she can see the top of the chimney in the mirror. She is then 2 m behind the mirror and her eyes are 1.5 m above the ground. Taking the two triangles formed at the mirror as similar, find the height of the chimney.

**Congruent**: same shape AND same size. All corresponding parts equal.
**Similar**: same shape, possibly DIFFERENT size. Corresponding angles equal; corresponding sides PROPORTIONAL (in fixed ratio k).

If two triangles are similar with scale factor $k$:
- Corresponding sides differ by factor $k$.
- Corresponding angles are equal (always, by definition).
- Areas differ by factor $k^2$ — critical fact, and a common test question.

**Examples**:
- Equilateral triangles are ALL similar (same angles 60°-60°-60°).
- A photograph and its enlarged print are similar.
- Two right triangles sharing one acute angle are similar (AA).