I assume these scenes where he’s younger and more energetic are flashbacks?
It’s not completely clear to me what’s happening. I figure he’s a prof or a TA tutoring a student in a chemistry class? Is that right? (It’s not clear to me because it’s never made clear just how the numbers they are writing down were arrived at. At first I thought he was having some kind of CSI fantasy about an investigation involving the stuff he was cleaning up!)
Do these flashbacks go somewhere later in the series? Or are they just to show us something about his past and his present attitude toward it?
(Do we know why a university professor ended up as a high school teacher? Not at all an unprecedented move but you usually want to know why it happened…)
Was glad to see Brian Cranston’s body language and intonation changed so fairly radically–before that point, he seemed (based on this show and Malcolm in the Middle) to be one of those actors who’s good at playing just a particular role.
It isn’t 100% explained what the situation was, it could have been Walt was an instructor (professor or etc) at a college and the girl was his student or TA. It could also have been two chemists in a research building (which would probably have some rooms that look like regular classrooms) and Walt was the elder / learned chemist.
The girl is actually a significant person from Walt’s past, given you probably aren’t looking for spoilers I won’t go into all that. I will say that girl does come up later in the series and some of their past together is explained, but not all of it and not specifically (a lot is left implied.)
Since it is never explained explicitly, I will say there is an implication that the discussion about the composition of the human body started a “line of thought” that lead to a company being formed later on.
It is explained explicitly later that they were graduate students together and collaborated extensively–their relationship is a plot point in upcoming episodes.
To answer the question, this is a philosophical discussion between peers, as the exact composition by elements of the human body isn’t all that interesting a research topic for chemists/biologists, nor something that would be needed in a class.
At the beginning, Walt and Jesse are sponging up what’s left of Emilio, while Walt(in flashback) is going over the list of chemicals comprising the average human body. Per the toin coss, Walt has to kill Crazy 8.
So, Walt is trying to see Crazy 8 as just a piece of meat to be disposed of, as a collection of chemicasls to be dissolved and flushed, but Crazy 8 insists of Walt, “Look at me.” Crazy 8, out of some survival instinct, knows he has to get Walt to see him as a likeable person in order to survive.
We know how that ends.
At the end (while he’s sitting in his car on the highway overpass), in flashback he and flashback-Gretchen come up short, and flashback-Walt’s saying, “Is that all there is? Something’s missing.” Gretchen suggests “the soul,” but Walt dismisses it as unscientific.
But hear-and-now Walt is no longer certain, having just choked a man to death with a bike lock, and, presumably, disposed of the body by chemical liquefication/dissolution, and flushing down the toilet.