Michael Phelps
Michael Phelps
[Megan and Cueball standing outside their en-Phelps-ified swimming pool.] Cueball: Why is Michael Phelps in your backyard pool? Megan: I don't know. He's been there all day. Go home, Michael! Michael Phelps: Woo! 18 gold medals!
[Megan and Cueball break out a pair of pool nets and unsuccessfully try to snag Phelps.] Cueball: Can you get him? Megan: He's so fast ! Phelps: Ha hah! Can't catch me! Splash splash
[Cueball heads off to fetch something.]
[Cueball returns with a hand truck full of Jello mix.] Phelps: Oh crap.

Michael Phelps is an American Olympic swimmer, who could easily be considered the best swimmer worldwide: he is the most decorated Olympic athlete of all time, with 28 medals, 23 of them gold (won in the 2004, 2008, 2012 and 2016 summer Olympics, so it would have been 18 Olympic gold medals at the time the comic was published). He was most dominant in the 2008 Beijing Olympics where he won gold in all of the eight events in which he competed, the record for a single games.
Cueball and Megan find that the Olympic medalist is in Megan's pool. He refuses to leave, and is too fast to be caught. Cueball brings in boxes of Jello Mix to fill the pool with, thereby gelifying the pool and trapping Phelps or forcing him to leave.
However, according to the title text, after having waited the time necessary for the water to gelify, Cueball realizes that Phelps has eaten all of the resulting Jello. This may be a reference to Phelps being used to eating impressive food quantities (about 12,000 calories daily) , to keep up with his strenuous exercise regimen; or it may be a reference to pictures of Phelps smoking from a bong that arose after the 2008 Olympics in Beijing, China, as Marijuana use is often associated with an increased appetite. Otherwise, the text may simply be a reference to Phelps being capable of achieving super-human feats, such as devouring an entire pool full of Jello.
Interestingly enough, just pouring Jello powder into a pool would not solidify the water into Jello. The water would have to be boiled, then quickly chilled, for the Jello to set correctly. As Randall is a scientist, he should have known this; therefore, it's possible that he purposefully ignored this fact in favor of the humor. Michael Phelps' top speed is also only around 2.3 m/s, which can easily be outrun by anyone on land.
[Megan and Cueball standing outside their en-Phelps-ified swimming pool.] Cueball: Why is Michael Phelps in your backyard pool? Megan: I don't know. He's been there all day. Go home, Michael! Michael Phelps: Woo! 18 gold medals!
[Megan and Cueball break out a pair of pool nets and unsuccessfully try to snag Phelps.] Cueball: Can you get him? Megan: He's so fast ! Phelps: Ha hah! Can't catch me! Splash splash
[Cueball heads off to fetch something.]
[Cueball returns with a hand truck full of Jello mix.] Phelps: Oh crap.
