Rum. That is all.Some alcohols can curdle it a little bit, Jack doesn't seem to. Makes it kind of foamy, though. I always get way too drunk with it and need to recover. I don't know what it is about it, makes me pull out all the stops.
Somewhat related, The Eggnog Riot of 1826 at Westpoint:
https://en.wikipedia.org/wiki/Eggnog_Riot