Diet Coke+Mentos
Diet Coke+Mentos
[Cueball and his friend are sitting on the ground, with a bottle of Diet Coke between them, and Cueball appears to be putting mentos into the bottle.] Cueball: This is the coolest thing. You just drop the Mentos in the Diet Coke... Friend: Uh huh.
[Diet Coke starts to fizzle.] Cueball: Give it a moment...
[Someone teleports into frame in a magic puff. Cueball's arms are raised, with a package of Mentos in one hand.] POOF
Friend: D-Dad? Dad: I'm back, son. We can be a family again.

This comic's premise is a reference to the phenomenon of dropping Mentos into a bottle of a carbonate beverage to create a geyser of said beverage . During 2007, a large number of videos depicting this phenomenon floated around the Internet.
Cueball wants to show this phenomenon to his friend, because it's "the coolest thing". However, instead of achieving the standard result, a geyser of Diet Coke, the friend's father magically appears (presumably from the dead or from abandonment).
An alternative interpretation is that the friend's father is resurrected/returned in an event unrelated to the geyser, undermining the geyser's coolness.
The title text shows that Randall considers (or at least then considered) this trick to be as cool as mixing corn starch and water to make a non-Newtonian fluid that reacts wildly with vibrations and impact.
[Cueball and his friend are sitting on the ground, with a bottle of Diet Coke between them, and Cueball appears to be putting mentos into the bottle.] Cueball: This is the coolest thing. You just drop the Mentos in the Diet Coke... Friend: Uh huh.
[Diet Coke starts to fizzle.] Cueball: Give it a moment...
[Someone teleports into frame in a magic puff. Cueball's arms are raised, with a package of Mentos in one hand.] POOF
Friend: D-Dad? Dad: I'm back, son. We can be a family again.
