I pick up a rock and throw it at a fast-moving target. Do I roll a DEX save, or an unarmed strike?

Question

For context, the rabbit happens to be host to the alien jackalope fungus that has just been vomited from a possessed NPC.

Cairn, page 11:

Saves: A save is a roll to avoid bad outcomes from risky choices and circumstances. PCs roll a d20 for an appropriate ability score. If they roll equal to or under that ability score, they pass. Otherwise, they fail. A 1 is always a success, and a 20 is always a failure.

Page 13:

Attacking & Damage: The attacker rolls their weapon die and subtracts the target’s armor, then deals the remaining total to their opponent’s HP. Unarmed attacks always do 1d4 damage.

Attack Modifiers: If fighting from a position of weakness (such as through cover or with bound hands), the attack is impaired and the attacker must roll 1d4 damage regardless of the damage die used during the attack.

Our group's been trying out this game, and enjoying it so far. However, the rules-light system leaves some questions unanswered.

In particular, I had been attempting to bean a rabbit with a rock. What should the Warden call for here?

In general, if an attack has a remote chance of connecting, does the PC:

1. Roll a save, then roll damage?
2. Roll damage with impairment, even if success is unlikely?

Answer 1

Die of Fate as an easy solution, Attack if you need more control over the chances. The Warden should figure out the details.

As with other rules-light games, go from the fiction and work with the GM. Searching for specific game mechanics is no help here. Instead, we use common sense with (probably) a little help from dice.

The Cairn rules say this explicitly:

Fiction First

Dice do not always reflect an obstacle's difficulty or its outcome. Instead, success and failure are arbitrated by the Warden in dialogue with the players, based on inworld elements.

Killing a rabbit is a mundane task. Don't focus on it. The Warden can allow an automatic success, assuming the PC is proficient in hunting. The Warden can ask for a Die of Fate roll, rationalizing that the weapon was improvised.

Roll a save, then roll damage?

According to the Cairn FAQ, a Warden can ask for a save as kind of an "ability check":

A PC or NPC should save only if one or more of the following conditions are met:

• When the outcome of an action is uncertain.

However, you never roll "hit" and then "damage" in Cairn. It's only one roll per attack:

A save is almost never required when a PC or NPC is under attack, as attacks hit automatically.

We can threat this situation as an attack if we want, although it is probably not worth the trouble. Give 1d4 for the rock and a couple of HPs for the rabbit for being "good at avoiding a hit", get 1/2 chance of success, tweak it in favor of one side if you think it would be more plausible, and viola.