Yeast Has a Narrow Comfort Zone

Active dry yeast wakes up fastest in water around 105-115°F (40-46°C) — warm to the touch but not hot. Push past about 120°F and the yeast starts dying, with die-off accelerating fast between 130-140°F (55-60°C), which is why a batch of dough that just won't rise is so often traced back to water that was straight from the hot tap. Water that's too cold won't kill the yeast, but it puts it to sleep: the rise just takes much longer, which is fine (and even desirable, for the deeper flavor of a slow, cold rise) as long as you're planning for the extra time rather than assuming the yeast is dead.

Sources & methodology: These tips draw on established food-science references — Harold McGee's food-science writing, America's Test Kitchen / Cook's Illustrated testing, Serious Eats' technique reporting, USDA food safety guidance, and peer-reviewed food-safety research (including UC Davis cutting-board studies) — not reworded recipe-blog consensus.