The process of producing the yeast most of us buy in stores creates a product with a hard protective coating that helps extend its shelf life, but also makes the yeast itself dormant and ineffective without activation via warm water. This activation process is known as proofing, and not just any temperature of water will do.

Yeast is sensitive to small fluctuations in its environment, and water that is the wrong temperature can kill your yeast or keep it from activating properly. The ideal temperature range won’t vary too much between brands, and for common active dry yeast, hot water between 105 and 115 degrees Fahrenheit should be used.

This temperature is hot enough to dissolve the dried yeast and wake it up, but cooler than temperatures that could kill it. Meanwhile, the temp range for instant yeast is between 120 and 130 degrees Fahrenheit, and fresh yeast can be activated at temps as low as 95 degrees Fahrenheit, since it doesn't have a coating to dissolve.