Chemistry
Difference Between Enthalpy and Entropy in Chemical Reactions
Quick fact
A reaction can be endothermic (absorb heat) but still be spontaneous if it increases the disorder (entropy) of the universe enough—this is why ice melts at room temperature even though it absorbs energy.
Why this is interesting
You've felt a cold pack get icy and a fire release warmth—but have you ever wondered why some reactions happen on their own even though they absorb heat, while others that release heat refuse to start?