Snapple
Snapple
[Above the frame:] This one is entirely James' fault.
[Two Cueball-like guys are standing and talking.] Cueball: Here, take a bite of this Snapple. Friend: food!
[Cueball's friend takes a bite.] Friend: Ow!  What is this? Clink
[The panel switches to Cueball.] Cueball: It's an apple infused with tin.
[Beat panel. A wide shot of the two.]
[Same scene, except the panel is lightly shaded and the is a box above saying:] Those of you who know your periodic table should be laughing right about now.

Cueball hands another Cueball-like guy an apple calling it a snapple. When the guy bites into it, his teeth go clink against the apple's metal surface; Cueball has infused the apple with tin.
The fourth panel is a silent wide shot, perhaps suggesting the joke was met with silence as a weak joke . As a meta-joke, the final panel might jokingly suggest that the silence is because those unfamiliar with the periodic table of elements don't get the joke.
The joke in this comic is pretty self-explanatory, especially given that the title text continues the trend in early xkcd comics of explaining the joke. Tin is a metallic element whose abbreviation on the periodic table is "Sn" (as the Latin word for tin is "stannum"). Thus, the apple is a "Sn-apple."
Snapple is a brand of beverages -- mostly bottled juices and teas -- whose name is based on a carbonated apple juice they once produced ("snappy apple").
James (in the caption) presumably once made a joke to Randall about tin or Snapple or both.
[Above the frame:] This one is entirely James' fault.
[Two Cueball-like guys are standing and talking.] Cueball: Here, take a bite of this Snapple. Friend: food!
[Cueball's friend takes a bite.] Friend: Ow!  What is this? Clink
[The panel switches to Cueball.] Cueball: It's an apple infused with tin.
[Beat panel. A wide shot of the two.]
[Same scene, except the panel is lightly shaded and the is a box above saying:] Those of you who know your periodic table should be laughing right about now.
