In "Extracts from Adam's Diary," Adam explains his annoyance at how Eve has incessantly continued to name things, including renaming The Garden of Eden to Niagra Falls Park. One of the ways Twain makes comic use of the diary is that Eve has knowledge of cultural references from his (Twain's) time. Adam has no clue what she's talking about. Adam also finds himself saying words like "we" and "she" which, if Eve had never been made, Adam would have no use for such words. In this story, Eve is the one who came up with personal pronouns and the differences of gender. This is actually an interesting philosophical (or anthropological) idea about the formation of language. At what point did "we" become a clear concept? 

Eve also is much more perceptive than Adam, warning him not to go over the falls. She also questions why certain animals (lions) would eat grass when their teeth clearly look like they're "designed" for eating other animals. 

All this seems humorous but these are actually quite practical questions which the allegorical story from the Bible does not cover. Twain is parodying this allegorical style but also being very clever and subtle in questioning the veracity of the story by supposing practical thoughts Adam and Eve would have had. 

Eve is constantly curious. Adam is content to be obedient. Adam notes that Eve told him she was tricked by the Serpent because of a miscommunication about the figurative term "chestnut" meaning an old joke. Here is another parody of the famous "Fall" being that it was not based on sin; rather it was based on a bad joke. 

Eve's diary is equally funny. She considers herself an experiment, referring to Adam as the other Experiment, as if God is a mad scientist. 

It is quite comical to read, but it is also introspective not just to consider what Adam and Eve thought but what others, thousands of years ago, thought as well. Eve writes how she made an awful but actually quite practical (in terms of survival) discovery. "That was because the fire had revealed to me a new passion--quite new, and distinctly different from love, grief, and those others which I had already discovered--fear. 

The story of Adam and Eve is ripe for comical and philosophical suppositions. The interest is in thinking how each made sense of the world and of each other. What makes it more interesting and hard to comprehend is that they, per the story, had no childhoods.