Today's post, Heat vs. Motion was originally published on 01 April 2008. A summary (taken from the LW wiki):

For a very long time, people had a detailed understanding of kinetics, and they had a detailed understanding of heat. They understood concepts such as momentum and elastic rebounds, as well as concepts such as temperature and pressure. It took an extraordinary amount of work in order to understand things deeply enough to make us realize that heat and motion were really the same thing.

Discuss the post here (rather than in the comments to the original post).

This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Angry Atoms, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.

Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.