It seems that Moose's grandpa did some odd jobs for a local catholic church affiliated with a convent, so he ended up doing a lot of Mr. Fixit-type work for the nuns. After awhile, he realized that something seemed odd in the large yard outside the Sisters' cozy home. There was something there that was just slightly abnormal--he could feel it. But he couldn't quite put his finger on what it was. It began to bother him so much that he began carefully scrutinizing the yard every time he was there, but it was not a terribly remarkable sight. There was a large white clapboard house with a large front porch, the paint beginning to peel and the windows sagging a bit. there was a gravel drive that went to the outbuilding at the back, and there were patches of grass and various prairie weeds competing for dirt space and sunlight all across the large, flat lawn. The edges of the lawn were simply fields (mostly corn back then, I suppose) and there were a few old scrap odds and ends leaning against the tired old house--tubs, pump-handles, things that don't rot away as fast as the wood scrap that gets left in the same places. He'd never been there on a day when laundry was not fluttering in large amounts on a set of long lines, even in the winter, but now that it was summer, that wasn't exactly a remarkable sight, either. The Sisters kept no pets, so there were no dogs, paths, or droppings to watch out for. What was the problem?
Now, Moose's grandpa had done many jobs in his time, and like most men of his generation around here, he spent some time in the mines as a young man. He finally settled on those clotheslines. Something just wasn't right there, but what could be the problem? They were just iron posts, smooth cords, and wooden pins. It took another visit or two before it finally occurred to him.
He was talking to one of the Sisters about some completely unrelated thing, as sometimes happens, when the conversation dropped away like a pair of sunglasses over a cliff, completely gone and irretrievable. The rope! It wasn't rope at all. It was "det cord," very similar to the stuff he'd seen used in the mines to set off larger charges simultaneously or to create cutting charges that followed lines and contours. And it probably wasn't the best possible choice for a clothesline, all things considered.
He inquired and was shown rest of the clothesline, which was just some old plastic-coated cord the sisters had found on an old wooden reel in the shed. Nothing special, and surely not some kind of explosive. In short, they appreciated his concern, but they weren't going to yank down their clothesline over nothing. Grandpa chose a small but sturdy tree on the back edge of the property, almost to the field, and looped the clothesline around the trunk three times about a foot off the ground. Then he moved all the nuns back to the road and detonated the clothesline (the method was not related when I heard this story, and I also have no idea whether det cord is strong enough to serve as an improvised clothesline, and if you insist on factual perfection you might be missing the point.)
The noise of the explosion distracted some of the Sisters enough that they didn't notice at first that the tree had been cleanly cut in two and toppled to the ground.
Brandon linked this helpful video showing what happens when you wrap det cord around a tree: