- [[Entropy]] of a closed system always increases - meaning, over time, less [[Energy]] is available for useful [[Work]] - another interpretation: it takes [[Work]] to have cold spread to hot - (naturally, hot spread to cold) https://www.youtube.com/watch?v=mGDJO2M7RBg&ab_channel=TheRoyalInstitution http://www.infocobuild.com/education/audio-video-courses/physics/Physics10-Spring2015-Berkeley/lecture-04.html