In "Blues Ain't No Mockin Bird," what does Cathy mean when she says she will write a story about the proper use of the hammer?
Cathy, in "Blues Ain't No Mockin Bird," is a third cousin with a special gift of understanding the unstated suggestions of life, for example, she knew why photographers who were photographing a man about to jump to his death would, "of course," reserve a few shots for later (for the aftermath of the event). The four children out playing saw Granddaddy come home from hunting with a chicken Hawk slung over his shoulder. They also saw its outraged mated swooping and attacking Granddaddy. They also saw Granddaddy nail the hawk to the toolshed door and then throw the hammer at the swooping, attacking hawk.
This scenario is what inspired Cathy to utter her proclamation that she will one day write a story about “the proper use of the hammer.” Since throwing a hammer is most decidedly not the proper use of a hammer, we can conclude that Cathy may mean "proper use of a hammer" literally. Since she also understates things that are not stated, she could also mean the symbolic or metaphoric "proper use of a hammer."
Granddaddy threw the hammer to stop the enraged hawk mate from attacking him. He then promptly, upon Granny's request, broke the cameraman's camera and told the filmmakers to get off their property. property that they owned.
It may be that Cathy is thinking of the "proper use of the hammer" as the metaphor of defending yourself and your loved ones against attack by using things in slightly unorthodox ways but to good effect, such as throwing a hammer at an attacking hawk and breaking a camera while politely explaining that intruders need to vacate the premises.