You don't buy lukewarm water, you heat water until its
lukewarm.
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The water in the bathtub was lukewarm.
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The swimming pool's water was lukewarm.
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Water is lukewarm when it is slightly above your temperature,
about 100-105 degrees F.
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The warmer of the two would be hot water. Tepid water is nearly
body temperature, so it is considered lukewarm. That means that hot
water is hotter than lukewarm water.