Cracker snaps contain silver fulminate i.e. creates the snap sound, a form of explosive that if taken in quantities, presents a significant risk to passengers, crew and the plane.
You will be asked to relinquish all dangerous or prohibited items found in your carry-on luggage. If you refuse:
AvSec x-ray bags going onto aircraft to ensure there is nothing dangerous in them. If something comes up on the x-ray that needs checking, the bag will be opened, searched and any dangerous good or prohibited item will be removed, with a bag search notification form left within the bag.