
1) all your sweat will evaporate
2) it's not humid
If you drink a hot beverage on a hot day your stomach's thermo-sensors become overactive and send signals to the hypothalamus that you're hot, causing that organ to trigger sweating. So, if it can all evaporate, you'll lose more heat than you took in when you drank the hot drink.
If it's humid and/or you're already soggy with sweat, choose something cold.
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