Gadgets used: Remote Electrical Charge
There is a total of twelve so-called Harley Heads inside the Sionis Steel Mill and destroying all of them will let you complete four riddles (three heads per each). Each time you find a head (H on the attached map), prepare the Remote Electrical Charge and aim at the heart. Successfully hitting it should result in the device exploding. Notice! If you want to listen to the short recordings beside each of the heads, be sure to make it before throwing the Batarang!