How does polystyrene keep the water warm?

Polystyrene is an effective insulator due to its low thermal conductivity, which minimizes heat transfer. When used to cover or contain water, it reduces heat loss by preventing the escape of warmth from the water into the cooler surrounding air. Additionally, polystyrene traps air in its structure, which further enhances its insulating properties. As a result, water remains warm for a longer period.