|Date||Feb. 10th 2010|
Complexion is the 700th xkcd comic.
Cueball suffers from acne. Like others afflicted with the same condition, he uses skin care products designed to treat acne. Unlike most other people, he does his own controlled trial by using them on only one half of his face and measuring the effects; the blemishes on the treated half of his face are noticeably diminished, while the untreated half remains the same, allowing him to isolate the effects of the cream versus the effects of time.
He convinces his friends to try the same experiment with different treatments so they can find out which works the best. In order to properly randomize the trials, he flips a coin. However, because half of his face is "scarred" (with acne blemishes), he's flipping a coin, and appears to be threatening someone with acid, he's mistaken for the Batman villain Two-Face. (Two-Face flips a coin to decide whether his victims will live or die, and was badly burned by acid.) Cueball is subsequently punched by Batman; the title-text implies this has happened before.
Salicylic acid and benzoyl peroxide are both chemicals known for their skin care effects (salicylic acid in particular is also used to treat psoriasis).
"Why do all my attempts at science end with me being punched by Batman? (P.S. benzoyl peroxide soap works great.)"