Answer:
The detail that would be best to include in a summary of the excerpt is:
a new way of thinking about gravity
Step-by-step explanation:
A summary can be defined as a short overview of a paragraph, an excerpt, an essay, a book etc. It is supposed to filter important information through, while leaving out any supplemental details that don't affect readers' knowledge of the main facts.
The passage we are analyzing here has several of those supplemental details that do not need to be included in a summary. Calling Einstein a "smarty-pants", for instance, is certainly not crucial to understand the main idea conveyed. The one detail that deserves to be included in a summary is "a new way of thinking about gravity." This detail reveals how Einstein's thinking led him to make astonishing discoveries and concoct a paramount theory. If it wasn't for that new way of thinking, physics would not be what it is today.