The trailer was cold again. Not the kind of cold that a coat fixes, but the kind that gets into your bones and stays there until spring, if spring ever comes.

I sat at the kitchen table with a cup of coffee that had gone cold twenty minutes ago and a stack of utility bills spread out in front of me like a hand of cards I didn't want to play. The heating bill was three hundred dollars. Three hundred dollars for a month of keeping the trailer from freezing, and we still froze. Maria said I should stop worrying about it, but she said that every month, and every month I looked at the bills and felt the cold and wondered how people in the world before us had managed without this problem.

Danny was in the other room, fixing the radio. He's seventeen and good with his hands, better than me anyway. I can fix a carburetor and change a radiator hose, but radios and computers and whatever the young people are doing with their electronics -- that's a different language. Danny speaks it fluently.

"Pop," he said from the other room. "Can you bring me the screwdriver? The small one."

I found it in the drawer where I keep the things I might need and probably won't. The small screwdriver, the one with the handle worn smooth by years of use. I brought it to Danny and stood in the doorway and watched him work. The radio crackled and then cleared, and he smiled -- that rare smile that still means something, even after seventeen years of marriage and a son who looks at me like I'm a broken appliance he's trying to fix.

"Thanks, Pop," he said.

"Anytime."

I should have gone to bed. It was past ten, and the trailer was cold, and tomorrow I had nothing to do but look at the utility bills and wonder how we were going to pay them. But I went to the backyard instead, where I had been working on something for the past ten years.

It started as an idea, the way ideas do -- something you hear at the bar or read in a magazine or think about while driving down Route 35 and realize you've missed your turn because your mind was somewhere else. In this case, it was a magazine article about geothermal energy, the kind of thing that catches your attention for five minutes and then disappears from your mind until it doesn't.

Geothermal energy works by tapping into the heat that's already in the earth. The deeper you go, the hotter it gets. The problem is getting that heat to the surface without spending more energy than you get out. The article described a simplified method -- a closed-loop system that circulates fluid through underground pipes, absorbs heat, and brings it back to the surface. Simple in principle. Hard in practice.

I started writing the calculations on the backs of utility bills. That's how I work -- on whatever surface is available, usually something I'm supposed to be throwing away. The back of a gas bill, the back of an electric bill, the back of a pizza box from the bar. Maria laughed at first, then stopped laughing, then stopped asking questions altogether. She's a practical woman. She knows that some people fix things and some people think about fixing things, and she's learned to respect the difference even when it doesn't pay the bills.

The prototype was simple: a copper coil wrapped around a steel pipe, submerged in a hole I dug in the backyard with a rented auger. The hole was sixty feet deep, the maximum depth I could reach without professional equipment. At the bottom of the hole, the earth was warm -- not hot, but warm. About fifty-five degrees Fahrenheit, the temperature of the ground at that depth. Fifty-five degrees in the middle of an Ohio winter is not much. But if you circulate fluid through the pipe and capture that heat with a heat pump, you can amplify it. Multiply it by four or five times. Enough to heat a trailer, maybe.

I built the prototype over six weeks, working after dinner while Danny played video games and Maria watched her soaps. I scavenged parts from junkyards, traded labor at the bar for tools, and spent three weekends at the hardware store buying copper pipe and fittings and insulation that cost more than I wanted to spend but less than the heating bill.

The first time I turned it on, nothing happened. The heat pump hummed, the fluid circulated, and the thermometer in the trailer went from forty-two degrees to forty-three degrees and then stayed there. I was disappointed but not surprised. I had known the first attempt might not work. Science doesn't work on the first try. Neither does anything else, really.

I adjusted the design. Changed the pipe diameter. Added more insulation. Increased the circulation rate. Turned it on again.

This time, the temperature went to forty-five degrees. Then forty-six. Then forty-seven.

I sat in the trailer with a thermometer in my hand and a cup of coffee in the other and watched the numbers climb. Forty-eight. Forty-nine. Fifty. It was not much. It was not enough to make us warm, not really. But it was something. It was proof that the theory worked, that the calculations on the backs of utility bills were not completely wrong, that a fifty-three-year-old man in a cold trailer in the middle of nowhere could build something that produced heat from the earth itself.

Danny came out to the backyard and stood beside me and watched the thermometer. "It's working," he said.

"Yeah," I said. "It's working."

"How much heat can it produce?"

"Not enough," I said. "Not yet. But I think I can get it there."

He nodded and went back inside. He didn't say anything encouraging, and I didn't want him to. Encouragement from your son when you're fifty-three and building a heater in your backyard is either pity or disbelief, and I didn't want either one.

A week later, a reporter from the local paper came to write a story. She'd heard about "the old guy building a heater in his backyard" from someone at the bar, and she thought it was cute. She was right about the cute part. The story ran on page six with the headline "Local Man Tries to Heat Home with Earth Power" and a photograph of me standing beside the prototype looking like what I was -- a man in a grease-stained jacket holding a wrench and trying to look like he knew what he was doing.

The story went viral. Not nationally viral, but locally viral. The kind of viral that means three phone calls from people who want to know if my heater works and whether they can build one too. One guy from Cleveland called and asked if I'd sell him the design. I told him the design was on the backs of utility bills and he could have them for free.

The calls stopped after a while. The story moved on to something else -- a local football game, a mayor scandal, a cat stuck in a tree. The world moved on, as it always does, leaving me with a prototype that worked a little better and bills that were still unpaid and a trailer that was still cold.

But it was better. Not much better. Maybe two degrees better. But better.

I kept working on it. Adjusted the pipe diameter again. Changed the fluid mixture. Added a second coil. The temperature climbed to forty-eight, then fifty, then fifty-two. It was still not warm. But it was warmer than it was before.

Maria made me coffee every night while I worked. She didn't say much, but she was there, and that was enough. Danny started helping on weekends, bringing parts from his job at the auto shop and asking questions that made me think about the design in ways I hadn't considered.

We never talked about whether it would work. We never talked about whether it mattered. We just kept working, because that's what you do when the trailer is cold and the bills are unpaid and the only thing you have is a hole in the ground and a theory written on the back of a pizza box.

One night in February, I stood in the backyard and watched the thermometer read fifty-four degrees. Inside the trailer, the air was maybe fifty degrees. It wasn't warm. It wasn't even comfortable. But it was four degrees better than it would have been without the prototype.

I thought about the earth beneath my feet, sixty feet down, holding its heat through winter and spring and summer and winter again, patient and indifferent and always there if anyone bothered to dig deep enough to find it.

I went inside and made coffee and sat at the table and looked at the utility bills and wondered how we were going to pay them. Then I picked up the small screwdriver and went back to work.