When making recipes that include yeast, you need to add a "lukewarm" liquid (usually water or milk) to activate the yeast. But how warm is lukewarm? And can you measure it without using a thermometer?
It is important that you get the temperature right since cold water won't get that yeast going, and hot water will kill it. Yeast is a leavening agent, what makes bread rise, so it needs to be alive before the dough is put in the oven (where the yeast dies due to the high temperature).
An active yeast converts the sugars in the dough into carbon dioxide, which causes the dough to rise and create bubbles after the dough has risen. And lukewarm water activates the yeast.
"Lukewarm water" generally means between 100 and 110 degrees Fahrenheit, 36.5 to 40.5 Celsius. If you don't have a thermometer handy, run the water on your wrist and if 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!) Just be sure as you are running the water that the temperature is maintained and it is not getting hotter.