In Jesus’ parable, the foolish man built his house on sand.
The image appears in Matthew 7:24–27, near the end of the Sermon on the Mount. Jesus contrasts two builders: the wise man builds on rock, while the foolish man builds on sand. When rain, floods, and winds arrive, the house on sand collapses, illustrating the consequences of hearing teachings without putting them into practice.
Matthew’s version emphasizes the building material and location, while Luke’s parallel account in Luke 6:46–49 describes a house built without a secure foundation. Both versions use construction as a metaphor for the strength—or weakness—of a person’s response to Jesus’ words.
The phrase has also become a common English idiom. To say that an idea, institution, or plan is “built on sand” means that it rests on unstable assumptions and is likely to fail under pressure. The contrasting “house on the rock” represents durability, obedience, and a sound foundation.