Teenager Andrea Cao came to the tank seeking $20,000 for 20 percent of her personal massage company Q-Flex, which makes a curved, plastic tool customers use on themselves to relieve back pain.
"It feels pretty good," said Cuban, after testing out the personal acupressure device.
While Robert Herjavec called Cao's pitch "a fantastic presentation," he and sharks Lori Greiner and Kevin O'Leary all passed on investing in the business, which had sold just 800 units.
When Mark Cuban discovered Cao had sold every Q-Flex by going door-to-door, however, he was instantly hooked.
"I love that," he said. "A capitalist is born!"
Known for not relying just on sales statistics, Cuban once again decided to trust his gut and invest in Cao alongside fellow shark Barbara Corcoran.
"I think you're a great role model," he said. "Barbara and I work together well. I can whip up a website and get it up and running."
Cuban and Corcoran offered $25,000 for 25 percent of the business, provided Cao could make time during school for a sales call.
"You have to be available, on call," Corcoran said, to which Cao replied, "Deal."
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