The Window That Cleans Itself

The rain had trapped them on the sunporch all afternoon. Soren had read the back of every label in Grandma Lena's basket of garden things twice.

"Listen to this one," he said. He was holding the white tube of sunscreen, the thick kind that turned your nose into a ghost. "Active ingredient. Titanium dioxide."

"That's the chalky one," Maya said. She had her face an inch from the new window, watching a drop crawl down the outside. "Grandma says it doesn't soak in. It just sits on top of you."

"That's the point," Soren said. "It doesn't soak in because it's not soaking up the sun into you. It catches the ultraviolet light before it gets to your skin. Then it lets it back out as heat."

"Heat." Maya turned around. "So the sunscreen is warm?"

"A tiny bit. The light comes in, the titanium dioxide grabs it, and instead of the light burning you, it leaves as warmth you can't even feel."

"Catches light and changes it," Maya said slowly. She liked that. She rolled it around. "Okay. So why does the window have the same word on it."

Soren stopped.

"What window."

Maya pointed at the little sticker still stuck in the corner of the new glass, the one Grandma hadn't peeled off yet. Soren came over and read it out loud.

"Self-cleaning glass. Photocatalytic coating. Titanium dioxide." He looked at the tube in his hand. He looked at the window. "It's the same stuff."

"The exact same word," Maya said. "On the thing that stops the sun and on the thing that cleans itself."

"That can't be a coincidence," Soren said, which from him meant he was already turning it over for the gear that connected them.

Maya put her finger on the glass where it was clean and clear. Outside, the old kitchen window two feet away had a green film of grime in its bottom corner, the kind that had been there since before either of them was born.

"This one's brand new," she said. "But Grandma's had it a year. She told me she hasn't washed it once. And look." She tapped. "It's cleaner than the old one. After a whole year of nobody touching it."

"So it really does clean itself," Soren said. "How."

They stood there. The rain ran down. Maya watched a sheet of water spread across the clean glass and not bead up at all, just lying down flat like the window wanted to be wet.

"Same stuff," she said again. "In the sunscreen it catches the ultraviolet light. So in the window it must catch the ultraviolet light too."

"But the window doesn't need protecting from sunburn," Soren said.

"No," Maya said. "So it does something else with it."

Soren got his notebook out of his back pocket. He drew the window. He drew a little arrow of sunlight coming down and hitting the coating. Then he stopped, pen up, because he didn't know what the arrow did next.

"In the sunscreen the energy comes out as heat," he said. "What if in the window it doesn't come out as heat. What if it does work instead. What if it uses the energy to break something."

"Break what," Maya said.

"The dirt." Soren's pen came down. "The ultraviolet light hits the coating, and the coating uses that energy to pull the grime apart. Like the light is a tool and the titanium dioxide is the hand holding it."

Maya went very still. "That's why it's wet like that," she said. "Soren. The water. It's not beading. On the old window the rain rolls off in drops. Here it spreads out into a sheet."

"So?"

"So a sheet washes. A drop just rolls. If the light breaks the dirt loose, and then the water spreads out flat and slides the whole broken layer off in one sheet, the window rinses itself every time it rains." She was talking fast now. "Light to break it. Water to carry it away. That's the whole machine and there's no moving parts and nobody has to do anything."

Soren stared at his drawing. The arrow of sunlight. The flat sheet of water. He added a second arrow underneath, the grime leaving.

"It needs both," he said. "The sun alone wouldn't clean it. The rain alone wouldn't clean it. It needs the ultraviolet light and the water together. That's why Grandma's never had to wash it. It's been sunny and it's been raining and that's all it ever needed."

Maya looked from the white tube to the clear window, the same six letters on both.

"The same exact stuff," she said. "On your nose it eats the light so it can't hurt you. On the window it eats the dirt using the light. Same powder. Somebody just pointed it at a different job."

"Same powder, different job," Soren said. He wrote it down. Then he wrote: what else is sitting around being one thing when it could be another.

"There's a whole list of those, probably," Maya said. "Stuff that already does something amazing and nobody's noticed yet because they only ever asked it to do one thing."

The rain came harder. The sheet of water on the clean glass thickened and began to slide, all of it at once, carrying a year of invisible grime down toward the sill in a single moving curtain.

Maya pressed her face close and watched the line of it travel.

"It's cleaning right now," she whispered. "It's working right now and we're just watching."

Soren turned the tube over in his hand one more time and held it up next to the window, and the chalky white sunscreen and the clear bright glass caught the gray storm light exactly the same way.