In 2010, engineer Philip Arnold was on his last day of his contract in IT support at the Frederick County Public Schools’ Career and Technology Center when school secretary Karla Eshelman asked him to substitute teach.

“She said, ‘Hey, I’m in a real bind. The teacher of CAD architecture and engineering is out on medical leave. . . . Will you substitute for one day?’ ” Arnold recalled. “I looked her square in the eyes and said: ‘Absolutely not. You are not going to put me in a room full of teens.’ ”

Arnold, then 49, agreed to help her on the condition that she find someone else for the following day. He’d spent his career as an engineer. It had never entered his mind to become a teacher. But at the end of that first day in the classroom, he realized he loved helping the students. He found Eshelman as she was scrambling to find another substitute and asked: “How do I stay?”

Ten years later, Arnold, 60, is still teaching. His skill in the classroom has earned him the title of The Washington Post’s 2021 Teacher of the Year, chosen from 17 finalists in D.C., Maryland and Virginia.

“I have absolutely loved every minute of it,” he said.

The Career and Technology Center, where Arnold teaches, provides career-specific training to about 800 students in 10th, 11th and 12th grade who apply from Frederick County high schools. Students spend half their day at the Career and Technology Center and half at their home high school. Arnold teaches just 20 students — 10 in engineering and 10 in architecture — five days a week for two years. Most will go on to study engineering and architecture in college.

It’s an intense experience, and Arnold said the key to keeping students engaged is letting them apply their learning to real problems that interest them. “Once you learn the technical skill, the real question — and this is what students just love — is what do you want to do with it?” Arnold said. “If you can open that door and think big, you really see some amazing energy.”

That approach has led Arnold’s students to create inventions such as submarines to robots, and it has taken them to exciting places, including MIT, Ethiopia and President Barack Obama’s White House Science Fair.

A student’s desire to provide clean water in developing countries, for example, led a team of Arnold’s students to Ethiopia. They were raising money to build a modification they had designed for the Hippo Roller — an existing invention that helps communities in rural Africa transport water — when Jo Elizabeth Butler, who runs a charity supporting children in Ethiopia called Ethiopian Children’s Appeal, heard their presentation at the Rotary Club of Frederick. She knew about an Ethiopian school that needed access to clean water. So, in 2017, a team of Arnold’s engineering students designed a rocket stove to clean the school’s well water. Part of the design’s genius was that it could be assembled from pieces of plate steel that could ship in a flat package, dramatically reducing the cost of shipping.

The team was chosen by the Lemelson-MIT Program as one of 15 high school teams to receive up to $10,000 to build its invention. The students presented it at MIT in June 2018. The following June, they traveled to Ethiopia to install it.

Other students choose to solve problems closer to home. Three are working on an invention to help the elderly father of one of Arnold’s friends. It’s a seat cushion that sits on top of a turntable. It plugs into a cigarette lighter and, using air hydraulics, rotates to make it easier for him to get out of his car.

Leila Cornejo, 16, a 10th-grader in Arnold’s engineering class, appreciates the autonomy that Arnold gives students. “He does really well in helping us take risks, because if we are ever stuck on something, he encourages us to ask questions,” she said. Whenever possible, he steps aside to let students help each other. “He calls on me when he knows it’s a problem I know how to solve.”

Before the pandemic, Arnold’s students worked so hard that he often stayed until 11 p.m., locking the building after the custodians left, because his students were in the lab.

Arnold experienced that same dedication from his beloved high school math teacher in Frederick County, Nancy Lewis. She let Arnold’s father drop him off at school an hour early, so Arnold could work with her in the math lab. They wrote a computer program together. “She was so excited when the computer spit out a response,” Arnold said, that “she ran down the hall to show the principal.”

Arnold’s students write code that he didn’t write until after college, but he says the sophisticated curriculum is less important than the way he makes students feel. “I really don’t think I’m special,” he said. “I care about them, about elevating them, about preparing them for the career that I’ve loved.”

For Arnold, sharing the love of his first career with students has made him fall in love with his second. “I wouldn’t change a thing about my life,” he said.