Watermelon...contains almost 92% water....since the specific heat capacity of water is too high....4200 J/kg K...therefore the specific heat capacity of watermelon is also high....i.e, it has a high capacity to absorb heat....hope i helped
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Higher the specific heat capacity of a body,the more heat is required to raise the temperature of a body. SO accordingly,
Water has a very high specific heat capacity. Water melon has a lot of water in it. Due to the presence of water in a water melon, it has high specific heat capacity and therefore thus does not heat up easily. It stays cool...
I hope this helps! 
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