Especially in baking, recipes usually say dissolve yeast in a “lukewarm” liquid (usually water or milk). How hot is that? Can I measure it without using a thermometer?
“Lukewarm” generally means between 98 and 105 degrees Fahrenheit, 36.5 to 40.5 Celsius. When you run the water on your wrist and it feels warmer than your body temperature, but not hot, that should be just about right. If you’ve ever tested the temperature of warm formula or milk in a baby’s bottle, that’s lukewarm!