Expandable post summaries are the method by which each blog post consists of a short intro paragraph followed by a "Read More" link. They make for faster viewing and loading of web pages while still giving readers access to the information they want. Turns out blogger makes it very easy to do this.
Using a feature called jump breaks or "after the jump" summaries, edit a post and in the "Compose" mode there is a icon that looks like this:
Insert the jump break after the paragraph or line where you want the introduction to end and everything below that line will appear once the user clicks the "Read More" link. That's all there is to it!