Poisoned apple
In short: The queen’s trap that Snow White bites without recognising the danger.
Object
How does it work in the tale?
In the Grimm telling, the queen eats the safe half. Her demonstration suggests the other half is safe as well. The apple carries the poison and provides a false reassurance.
What changes with the apple?
The queen first brings laces and a comb. The dwarfs revive Snow White by removing them. The piece she bites from the apple stays lodged inside, leaving the dwarfs with a problem their earlier remedies cannot solve.