Mostly the potato, I'd say... as it will probably contain potentially toxic copper salts after the operation. If the adapter generates 9-12V, it will be pretty

Mostly the potato, I'd say... as it will probably contain potentially toxic copper salts after the operation. If the adapter generates 9-12V, it will be pretty unpleasant to have that voltage in your mouth, but probably not dangerous (like licking a 9V battery to check if it's still charged...)