The name hammerhead describes the animal's wide,flat head.
VOA: special.2010.08.03
People driving during a tornado are told to find low ground and lay flat,facedown, with their hands covering their head.
VOA: special.2011.04.19
When she returns the screwdriver a couple days later, your friend mentions that she made an improvement: now it works with both Phillips and flathead screws.
The family later noticed that the back of her head was flattened, and she was diagnosed with severe plagiocephaly, sometimes referred to as 'flat head syndrome', by a specialist clinic in Leeds.