Snapple
Sn = tin
Sn = tin
Snapple is a brand of beverages that mostly sells bottled juices and teas. Its name is based on a carbonated apple juice the company once produced ("snappy apple"). In this comic, Cueball hands another Cueball-like guy an apple calling it a "Snapple". When the guy bites into it, his teeth are blocked by the apple's metallic surface because Cueball has infused the apple with tin. Tin is a metallic element whose abbreviation on the periodic table is "Sn" (as the Latin word for tin is "stannum"). Thus, the "tin apple" could be referred to as a "Sn-apple". 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. According to the caption at the top, James Zetlen presumably once made a joke to Randall similar to the one in the comic.
The title text continues the trend in early xkcd comics of explaining the joke rather than expanding on it.