Final answer:
The subject of this question is Water Conservation, which emphasizes the importance of using water wisely.
Step-by-step explanation:
The topic of this question is related to Water Conservation in the context of science. Specifically, it is about the limited availability of water and the need to use it wisely.
Water is a vital resource, and we need to be mindful of how we use it. The statement in the question suggests that water can only be used once in the morning and once in the afternoon, which highlights the importance of conserving water.
Water scarcity is a growing concern globally, and it's crucial to make responsible choices to ensure we have enough clean water for the future.