Zach Quinn and Brian Keller already have learned Rule One about building a business.

Break the rules.

Quinn, 20, and Keller, 19, are sophomores in Jay Ebben's Foundations of Entrepreneurship course at the University of St. Thomas. As he does every semester, Ebben issued his fall 2012 students his Lemonade Stand challenge. Team up and get to work creating a brand built on the lemonade-stand model, including a business plan and revenue stream.

You have three months.

"You're asking us to do what?" most students ask Ebben. "This is the first course undergraduates take as they consider building a business from scratch," he said. That's why he advises them to think small.

Quinn and Keller didn't listen.

Inspired by the altruistic buy-one-give-one model created by TOMS Shoes founder Blake Mycoskie (www.toms.com), Quinn and Keller looked up instead of down. They designed a line of 100 percent cotton hats for kids undergoing treatment for cancer.

"We talked to hospitals and found out that people donate big boxes of hats for kids, but they don't always fit," Quinn said. "We wanted to design a product that anyone can wear." And feel really good wearing. They envisioned hats in fun, bright colors -- turquoise and violet, forest green and gold.

They decided that every part of their production, from the knitting (Columbiaknit of Portland, Ore.) to the sewn-on logos (Advantage Emblems of Duluth), would be made in America.

And they needed a fun brand name. Love Your Melon (loveyourmelon.com) is a nod to our noggins, and to the original lemonade assignment.

After many late nights brainstorming and refining, Quinn and Keller secured $3,500 in startup funds from family friends. Class guidelines said to keep projects under $700.

Then they got to work getting 400 hats produced (200 to sell for $20 and $25, and 200 to give away). Ebben strongly suggested they start out with half that many.

They sold out in three days and have ordered 800 more.

"We sold 50 to complete strangers," said Quinn, who carries a yellow steno pad with him everywhere. "We sold others on Facebook and about 60 on campus." A few made their way to Iowa and Illinois.

Then the fun began -- giving away the other half. Last week, they gave 45 hats to children at the University of Minnesota's Amplatz Children's Hospital, assisted by Santa Claus, plus another 25 to children at the Ronald McDonald House and 30 to individual families.

They'll return to Amplatz on Tuesday to give away 100 more, with the help of the Minnesota Vikings.

"They're great guys to be as young as they are and this interested in giving back," said Jim Leighton, a spokesman for the Children's Cancer Research Fund, which raises money to support research at the U. Choosing hats, he added, was especially thoughtful.

"To lose their hair is really an emotional part of the journey," Leighton said of the pediatric patients. "They have a rainbow of colors to choose from, and the hats are wrapped in a ribbon. It's a gift."

Ebben laughs at his original advice. "What's also funny is that we get each team a mentor who's an alumni of the program. Brian and Zach's mentor also called me and said, 'I don't know about these guys buying 400 hats.'"

Keller graduated from Andover High School and was considering a career in mechanical engineering, but everyone told him he was going to end up in business. Quinn graduated from Mounds Park Academy. He spent a year at the University of Denver, then returned to the Twin Cities to enroll in St. Thomas' entrepreneurship program.

He and Keller met at the university and quickly realized they had different strengths that would work well together.

"Brian is one heck of a salesman," Quinn said.

"Zach is incredible with details," Keller said. "He also really emphasizes quality in everything he does."

The full-time students already have made a profit, but they're pouring it all back into the company, something they're working on constantly when not in class or studying.

"We're barely sleeping, especially with finals going on," Quinn said. "We love what we're doing, the lessons we're learning."

Ebben, who began the Lemonade Stand challenge a year ago, also had high praise for Keller and Quinn's classmates. One group wrote a "beautiful" children's book about a child who climbs into a lemon tree. Another group made jewelry out of dried lemon peels. Yet another created a comedy troupe on campus: Lemonade Stand-Up.

"Really neat stuff," he said. "It's been a lot of fun watching them run with their ideas."

Quinn and Keller, though, got top marks for clearly articulating their vision and selling that vision to others.

"It really is extraordinary," Ebben said. "It came together in such a short period of time and is far beyond what we expected from the class."

gail.rosenblum@startribune.com 612-673-7350