Sn = tin
In reality, Snapple is a brand of tea and juice based beverages whose name is based on a carbonated apple juice they once produced ("snappy apple"). The joke in this comic is pretty self-explainatory; especially given that the image text continues the trend in early XKCD comics of explaining the joke.
This comic features two Cueball characters (let's call them "lefty" and "righty"). Righty offers lefty a "snapple" which looks (in black and white) like a normal apple. Lefty replies “Food!” suggesting he either has an addiction to food, or he is very hungry. When lefty bites the snapple, it makes a metallic “clink” and righty tells him that the apple is infused with tin. Tin is a metallic chemical element whose abbreviation on the periodic table is "Sn" (as the Latin word for tin is "stannum"). Thus, the apple is a "Sn-apple"
The third panel is a silent wide shot, perhaps suggesting the joke was met with silence as a lame joke. As a meta joke, the final panel might jokingly suggest that the silence is because those unfamiliar with the table of elements don’t get the joke.
It is unclear who James (in the caption) is, but presumably he once made a joke to Randall about tin or Snapple or both.
- This is another early XKCD comic in which Cueball characters are drawn in some panels with faces.