Give Rustie the milk first, and only then the dead mouse.
Official in-game clue: "Kitty wants something. What do cats love?"
Where: Day 1 Morning, in the hallway, while you are collecting the porridge ingredients.
Why it gets missed
You are carrying a dead mouse and a bottle of milk, and a cat is in the room. Handing over the mouse is the funnier and more obvious move, and it works — the cat takes it, the scene advances, and nothing tells you an opportunity just closed.
The clue is asking a question with one answer: what do cats love? Milk, not vermin.
What to do
- While gathering wood, oats, milk, a small pot and matches for the porridge, keep the milk.
- Give the milk to Rustie, the orange cat.
- Then give him the dead mouse.
Both are needed; only the order is graded.
The milk is also an ingredient
Do not panic about spending it — the game hands you what the recipe needs. Full ingredient list and cook order on the porridge page.
Rustie turns up again on Day 3, asleep on the maid's lap, for a second and much simpler achievement.
Nearby
The full list of hidden achievements, in the order you reach them, is on the triggers page. Going for everything in one run? See 100% completion.