Entropy

Entropy is a count of how many ways a system can be arranged while still looking the same from the outside. More possible arrangements means higher entropy. A tidy room has few arrangements: every book has one right spot. A messy room has astronomically many arrangements that all count as 'messy'. That's why rooms drift toward mess on their own but never tidy themselves: there are simply far more messy configurations than tidy ones, so random shuffling almost always lands on messy. Heat works the same way. Energy spreads out because 'spread out' has vastly more microscopic arrangements than 'concentrated'. The second law of thermodynamics is really just probability at a huge scale.