In "The House of Unexpected Sisters" by Alexander McCall Smith, a character reflects on the finality of death through a poignant metaphor. He describes his old friend who has passed away, signifying the irreversible loss and the stark realization that what seemed innocent, like water (H2O), was, in fact, something deadly like sulfuric acid (H2SO4). This imagery captures the essence of misunderstanding and the tragic consequences it can lead to.

The quote emphasizes themes of friendship, loss, and the painful truth that life can hold hidden dangers. The character's contemplation serves as a reminder of the fragility of life and the importance of understanding the reality behind appearances. This moment poignantly illustrates how a simple mistake can lead to devastating outcomes, making us reflect on the nature of relationships and the potential perils that may lie beneath the surface.