Hair Growth Rate
Hair Growth Rate
[White Hat and Ponytail are walking to the right.] White Hat: The average head has about 100,000 hairs. White Hat: And hair grows at about ½" per month. White Hat: Plus or minus. Ponytail: Okay...
[They continue to walk while White Hat lift a hand up palm up.] White Hat: So our heads are producing an inch of hair every minute. Ponytail: I see.
[They continue to walk.] Ponytail: I'm just glad it's evenly distributed. It would suck if we grew a single new five-foot-long hair every hour.
[White Hat and Ponytail are seen in silhouette from a distance. White Hat has lifted a finger up and while Ponytail has thrown both her arms out to the sides.] White Hat: Hmm, would the hair grow steadily, or would it suddenly shoot out 5 feet on the hour? Ponytail: If the latter, what noise would it make? Ponytail: Ziiip? Pwiff? White Hat: Fwip? Ponytail: Blip. White Hat: Zhooop. Ponytail: Pew!

This strip is one of the simpler jokes that xkcd has done, being an observation on mathematics, biology, and human expectation. White Hat starts by sharing various facts about hair with Ponytail; hair count, individual hair growth rate, and finally total hair growth rate. Ponytail proceeds to snark about how unpleasant it would be if, rather than 100,000 hairs growing at a gross total of five feet (1.524m) per hour, humans grew a single hair at five feet per hour. The comic then delves into the absurdity of gradual versus spontaneous growth, and then the sound effects involved therein.
The comic touches on what information can be obscured by just looking at aggregate values.  A person whose 100,000 hairs grow a half-inch (1.27cm) per month experiences the same total new hair growth as a person with one hair growing five feet in an hour, but their grooming experiences would be very different.  Likewise, a person with one hair growing steadily for an hour has the same average rate of hair growth as a person experiencing sudden hair growth on the hour, but the profile of instantaneous energy conversion and protein production would be very different.  One of Ponytail's suggestions for what five feet of instantaneous hair growth might sound like is a sound effect generally used for directed-energy weapons ( Pew! ).
We never see what sort of hairstyle White Hat has under his hat, but Ponytail's hair is fairly long.  If she had to grow it out by one hair per hour, as in the title text, then it would take over eleven years before all 100,000 hairs had grown out.
[White Hat and Ponytail are walking to the right.] White Hat: The average head has about 100,000 hairs. White Hat: And hair grows at about ½" per month. White Hat: Plus or minus. Ponytail: Okay...
[They continue to walk while White Hat lift a hand up palm up.] White Hat: So our heads are producing an inch of hair every minute. Ponytail: I see.
[They continue to walk.] Ponytail: I'm just glad it's evenly distributed. It would suck if we grew a single new five-foot-long hair every hour.
[White Hat and Ponytail are seen in silhouette from a distance. White Hat has lifted a finger up and while Ponytail has thrown both her arms out to the sides.] White Hat: Hmm, would the hair grow steadily, or would it suddenly shoot out 5 feet on the hour? Ponytail: If the latter, what noise would it make? Ponytail: Ziiip? Pwiff? White Hat: Fwip? Ponytail: Blip. White Hat: Zhooop. Ponytail: Pew!
