The Truss

Part One · The Pretty Bridge

The county bridge contest had three rules, and Marisol Ybarra had copied them onto an index card that she kept propped against the lamp on her desk. Every bridge had to be constructed from no more than two hundred craft sticks and ordinary white glue. It had to span a gap of forty centimeters between two tables. And on contest day a bucket would hang from the middle of it while a judge poured in sand, one scoop at a time, until something gave way. The winning bridge was displayed in a glass case in the lobby of the county library for a whole year, with the builder's name engraved on a brass plate beneath it. Marisol had inspected last year's winner every Saturday since September, and although it was strong, apparently, but it resembled a heap of fencing, and she was certain she could do better.

Her bridge was wide and flat and painted the turquoise of a swimming pool, with railings along both sides and a row of miniature lamp posts made from toothpicks and beads. The sides were a sequence of tidy square frames, like the windows of a train, which she considered elegant. She assembled it at the kitchen table across three weeks of July, while her cousin Benji, who was seven and staying with them for the summer, constructed things of his own at the opposite end. Benji's creations were made of toothpicks and gumdrops and nothing else, and they were invariably made of triangles, because his teacher had informed him that triangles were the strongest shape. His latest tower had been supporting two cookbooks and a dictionary since Tuesday. Marisol said that was nice, and relocated her glue out of his reach.

Two weeks before the contest the library held a practice evening, and Ms. Fairweather from the county roads department brought her own bucket and a sack of sand, and Marisol's bridge was the fourth one tested. It looked magnificent on the tables, turquoise and level and faintly glossy under the lights, and for the first two scoops nothing happened at all. On the third scoop the square frames along one side began to lean, all of them simultaneously, like a row of people deciding to sit down. The deck tilted, the railings twisted, and then the entire bridge folded sideways with a soft crackle and deposited the bucket on the floor. It had held a little less than four kilograms. Ten minutes later a boy named Cormac positioned his ugly gray bridge across the same tables, and it held twenty-one.

Marisol did not utter a single word in the car on the way home, and her father, who was wise about such things, did not ask her to. In her room she laid the wreckage on the desk under the lamp and examined it for a long time, because something about it puzzled her. Hardly any of the sticks had snapped, and the glue had held at nearly every joint. The pieces were all still there, precisely the length they had always been, and yet the structure had collapsed, as though the bridge had simply changed its mind about what shape it intended to be. She could not comprehend how a bridge could fail without breaking, and she could not comprehend what Cormac's ugly bridge knew that hers did not, with thirteen days remaining before the contest.

Part Two · Squares and Triangles

In the morning Benji was installed at the far end of the kitchen table with his gumdrops, and Marisol sat down opposite him and asked him to demonstrate how his towers worked. He regarded her suspiciously, as if it might be a trick, and then he made her a square out of four toothpicks and four gumdrops and stood it upright on the table and instructed her to push the top corner. She pushed it with one finger, and the square leaned over into a diamond, smoothly and effortlessly, without a single toothpick bending. Then he made a triangle and told her to push that. She pushed it, and then she pushed harder, and the triangle did not alter its shape at all. It slid across the table in one piece, still exactly a triangle, until it collided gently with the fruit bowl.

It took her most of the afternoon to work out why, with Benji supplying gumdrops and her father supplying a pencil. If you have three sticks of fixed length, there is only one triangle they can possibly form, so its corners cannot move unless a stick stretches or snaps. Four sticks can form a square, but they can equally form a tall diamond, or a flat one, or any shape in between, because the corners swing like hinges. That was precisely what had happened at the library: her square frames had not broken, they had merely swung. After supper her father took her up into the attic with a flashlight and showed her the wooden rafters, and every one of them, from one end of the house to the other, was braced into a triangle. He said builders called that framework a truss.

Once she knew to look, she noticed trusses everywhere, which was slightly irritating. The old railway bridge over Pike Creek, which she had walked beneath a hundred times on the way to the swimming pool, was a long box of steel triangles. So was the crane at the construction site beside the school, and the power tower behind the soccer field, and, more or less, the frame of her own bicycle. Every structure that had to carry weight seemed to have quietly agreed upon the same secret, and that evening she sketched a new design with two long sides made of triangles, like a row of tents, and diagonal braces crossing in an X beneath the deck, so the floor could not twist the way the old one had. It had no lamp posts. It was not as pretty, and she minded that more than she would admit.

She asked Benji to be her assistant, and he accepted the position with tremendous solemnity. His responsibilities were to sort the sticks by straightness, because a warped stick made a feeble side, and to hold each joint perfectly motionless for a whole minute while the glue grabbed. He was extraordinarily good at holding still, it emerged, which nobody in the family had previously suspected. They built the two sides flat on waxed paper, one triangle at a time, then stood them upright and connected them with the braces underneath. When the first side had dried, Marisol suspended Benji's backpack from the middle of it, crammed with library books, and it did not lean, or wobble, or so much as creak. Benji pointed out that this was what he had told her in July. Marisol said yes, it was, and thanked him, which astonished them both.

The night before the contest she painted it, because she could not help herself: not the whole bridge, which would add unnecessary weight, but a narrow stripe of turquoise along the top of each side, so that from a distance the triangles seemed to ripple like water. She deliberated about the lamp posts for a while and concluded that some things mattered more than others. Then she counted the sticks, and there were one hundred and ninety-four. Benji wanted to know what would happen if the new one collapsed too. Marisol told him honestly that she did not know, that it might, and that if it did they would study the pieces and work out why, exactly the way they had last time. He seemed satisfied with that answer, although she was not entirely convinced that she was.

Part Three · The Bucket

The contest took place in the exhibition hall at the county fairground, which smelled of popcorn and floor polish, with forty bridges arranged along trestle tables and a pair of sawhorses at the front where the testing happened. Benji carried the bridge from the car in both arms, slowly and ceremoniously, like a birthday cake. Cormac was there with his gray one, and he glanced at Marisol's triangles and nodded, as though she had finally caught up with something obvious, and she decided not to mind. When her name was announced she set the bridge across the gap herself, checked that it sat level, and stepped back with her hands clasped behind her, the way people stand in front of paintings in museums. Ms. Fairweather hooked the bucket to the middle and picked up the scoop.

At four kilograms the old bridge had folded, and at four kilograms the new one did nothing whatsoever. At ten it emitted a small dry tick that made Benji seize her arm, and at fifteen the deck sagged by the width of a fingernail and remained there, perfectly steady, while the hall went quiet around them. Ms. Fairweather continued pouring. At twenty the triangles along the sides were still exactly triangles, crisp and rigid, and at twenty-five people at the back were standing on chairs to see. It finally gave way at twenty-eight and a half kilograms, with a noise like a bundle of pencils snapping, and the bucket landed on the floor with a thump. The sides had endured to the very end; it was one of the glue joints underneath that had surrendered first.

Cormac's bridge held twenty-six, which meant that hers had won, and he was the first person to come over and congratulate her. He wanted to know how she had managed the bracing underneath, so she illustrated it on the back of the index card, and he confessed that he had used squares beneath his and would not be repeating that mistake. When Ms. Fairweather asked for the name for the brass plate, Marisol spelled out two names, and the second one was Benjamin Ybarra, assistant engineer, age seven. The broken bridge went into the glass case at the library, sand and all. At home, on the kitchen windowsill, Benji's gumdrop tower was still supporting two cookbooks and a dictionary, and Marisol hung the blue ribbon from the very top of it.