The hallway outside the engineering classroom at Stillwater Area High School became a runway on a recent morning for a special guest testing out some new gear crafted by students.

With some encouragement, Sadie, a three-legged brown-and-white Brittany spaniel, took a few hesitant hops before taking off down the corridor in a makeshift cart of PVC pipe, swivel wheels and a few sponges for added padding.

The contraption was one of many prototypes of carts or prosthetics — most made with cardboard, Velcro, duct tape and some 3-D-printed parts — that Stillwater High's engineering students have designed and created to help their furry friend. The project has given them hands-on experience and tested their math and computer skills to solve the problem.

"I love this. I absolutely love this," said Sadie's owner, Nancy Schoenecker, a special education paraprofessional at the school. "The kids love her. She loves the kids. It's really cool to see what they're coming up with."

Now eight, Sadie has lived most her life with just three legs — her right front leg was amputated several years ago after she ran out of the yard and was hit by a car.

"For a long time she was just as fast on three legs as she was on four, but that's changed," Schoenecker said as she snuck Sadie a bite of string cheese in between prototype tests.

Dogs carry most of their weight on their front end, and arthritis in Sadie's remaining front leg is taking a toll. She's often so wiped out by late afternoon she starts falling and tipping forward. In the evenings, Schoenecker has to carry Sadie outside and to her bed.

A conversation with a student teacher led her to Matt Howe's engineering classes. Howe is often looking for real-world problems he can have students design solutions for, and Sadie seemed like the perfect candidate.

Still, Howe admits that "Project Saving Sadie" was daunting. Once his students met the dog, however, they were motivated.

"Anytime you bring in that empathy factor, the kids are into it," Howe said. "Even the tough guys are like, 'Aww, a three-legged dog!' "

Howe said the project, which has stretched for a few weeks, has challenged the students in ways that an online project or a textbook never could. He repeatedly reminds his classes that learning engineering skills is often more about the process than the product. And he encourages them to reflect on how far they've come since the beginning of the course when modeling a 2-by-2-inch cube using computer-assisted design.

"Some of you are going to look at your design on Sadie and go, 'What was I thinking?' " Howe said at the start of class before Sadie tested the prototypes. "But that's OK. That's how you learn and then you modify."

Junior Greg Hoffer watched Sadie walk with his design — a sort of peg leg attached to a Sadie-sized dog shirt.

"It could have been better," he said after seeing how the new leg flopped around when Sadie moved forward.

"We definitely learned that just because you think something is going to work flawlessly doesn't mean it will," Hoffer said. "But that's why we're here. We're problem-solving for Sadie."

Almost immediately after sending Sadie down the hallway in the PVC-pipe cart, its designer, sophomore Christian Harvel, noted the modifications he could make to the cart and its wheels to keep it from turning and tipping.

"It made me really happy to see [Sadie] excited to move around," Harvel said. "We've learned that we can design something that could be helpful in bringing a three-legged dog's old joy back."