A challenge where you put a bunch of mentos in your mouth and trying to consume a 2 liter bottle of diet coke spilling as little as possible. The chemical reaction produced by the coke interacting with the mentos causes it to burst out causing people to spill coke while consuming it.
"Chester and I tried the Diet Coke and Mentos Challenge last night, man it hard to chug down that Diet coke tho!"
The act of inserting a full package of mentos to the inside of an asshole during anal sex and - subsequently - adding your jizz and a glass of diet coke to the mix.