Learners investigate to see if yeast needs warmth to grow. Sugar and warm water (37 degrees Celsius) are food for yeast. Yeast is alive. The dry yeast that you find in packets, and jars is still "alive". But it's "Inactive" (basically "suspended animation") If the water used to reactivate it is too cold it won't thrive... Just like hyporthermia affects humans in a negative fashion, so does yeast in cold water. You can effectively kill it by using very cold water. But most tap water won't kill it... But will keep it "inactive".