"You have to make them see how it affects them right now and not worry when they're going to see it again," he said.
Plus, teaching groups of sophomores and juniors the laws of gravity as opposed to teaching second and third graders "what goes up must come down" just isn't the same.
"It takes a little more to impress them," he said. "A lot of times they're impressed, but they don't want to admit it."
Johnston said he'll start off the class with a flashy demonstration to catch students' attention.
He's shown students how to make mousetrap cars, how to implode a soda can and how to explode just about anything.
He'll put copper wire, a magnet and two poles on a base in front of the students and ask them to make it work like a generator.
"They want to find out how things work," he said.
Lessons can't always be taken outdoors, but when the classroom does venture outside, he'll take students to familiar territory - the car.
Johnston said he uses his own car for many demonstrations. Changing a tire turns into a lesson on torque, rotation on an axis, but serves a dual purpose.
"How many of them will remember it, I don't know," he said.
Johnston took his physics classes to an amusement park last year to study the momentum, velocity and energy transfers of roller coasters. He said he gave the students worksheets to fill out while they went around to different rides.
"I still have a lot left to learn, and I learn a lot from the teachers around me," he said.
He listed fellow Hancock science teachers, Jim Hoopengardener and Carl Wise, as his scientific idols. Johnston said he also favors his former professors from Indiana University of Pennsylvania to famous scientists.
"I'm into science more than the history of science," he said.
He offers incentives to students, like giving them bonus points if they bring in clippings of science-related comic strips, because it reminds them that science is everywhere.
Before Christmas, Johnston showed students how different tree lights work on different electrical currents.
He said it's cheaper to wire tree lights so if one bulb fizzles out, the others follow.
Johnston isn't sure what sparked his desire to teach high school science.
"I don't think there was a moment. I think it was a process over time," he said.
Johnston said when he was in school he helped friends learn difficult concepts, which may have foreshadowed his career choice.
He doesn't see himself in the same role now, though.
"I do care about them, but I can't consider them friends," he said. "It's a fine line - one of the most difficult lines in teaching."