This is because water has a specific heat value much higher than the specific heat value of sand.
Explanation:
The specific heat of a substance refers to the amount of heat required to change the temperature of a substance by 1ºC. Each substance has its own specific heat value and the higher this value, the more difficult it will be to change the temperature of that substance.
The water has a very high specific heat value and much higher than the specific heat of the beach sand. In this case, even though they are heated by the same amount of solar energy, the sand gets much hotter than the water, because it takes time for the water to change in temperature.
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