Buildings that aren't water-resistant

SANDCASTLES

["These temporary structures are often found on beaches and washed away by waves."]

The clue plays on the literal meaning of "buildings" and the idea that sandcastles aren't water-resistant—they easily wash away with water. The word "buildings" is a straightforward definition, while "aren't water-resistant" hints at the fragile nature of sandcastles. Together, these elements lead to the answer "SANDCASTLES" through a mix of direct description and playful reasoning.