#3028: D&D Roll

XKCD comic, described below.
Transcript

[Cueball, Megan, Ponytail, White Hat, and Knit Cap are sitting around a table in a tabletop gaming session. Both Cueball and Knit Cap are sitting in office chairs at the ends of the table, with Cueball leaning forward and holding his hand above the table and Knit Cap leaning back on her arm. Behind the table, Megan sits to the left of Ponytail and White Hat to the right. They are both looking at Ponytail, while Ponytail is looking at Cueball. Objects such as dice, miniatures, a map, and papers are on the table.]

Cueball: I roll D20... 18.

Ponytail: The kobold is unaffected.

Ponytail: Honestly, I don't know why you thought dice would help. You should probably try a sword or something instead.

(Sourced from explainxkcd.com)

Under some circumstances, if you throw a D8 and then a D12 at an enemy, thanks to the D8's greater pointiness you actually have to roll a D12 and D8 respectively to determine damage.

ExplainOriginal