Three steps, and almost everyone stops after the first.
- Use the magnet on the hole to pull out the key.
- Click the hole again. This draws the mouse out.
- When it runs back across, hit it with the sledgehammer.
The key is what you came down here for, so step one feels like the end of the puzzle. It is not — the achievement is in steps two and three.
Finding the hole
Take both tools with you: the hammer and the magnet. At the fork, go left — the mouse hole is on that side.
If you arrived without the hammer, you will get the key and nothing else, and the block will close behind you.
The achievement
Pest Control. The official clue is unhelpfully terse:
"The hole is inhabited. Use the sledgehammer!"
It tells you the tool and says nothing about having to make the mouse appear first, which is exactly why players collect the key and walk away thinking the hammer was for something else.
Timing the swing
The mouse does not sit still to be hit. It comes out, crosses, and comes back — swing on the return pass. Missing is not fatal; you can draw it out again.
Where this sits
Day 2 Night, in the cellar, during Inspecting the Cellar. It is the last interaction of Day 2.
Before this, on the balcony, Garbage Disposal wants you to throw everything over the edge — clue: "This is disgusting! Throw them down! The balcony isn't a trashcan!"
Full day on the Day 2 walkthrough. Next up is Day 3 and the alchemy.