Self-made billionaire and "Shark Tank" star Mark Cuban has been hustling since he was a kid. One early business scheme, collecting and re-selling valuable stamps, even earned him enough to help pay for college.

"I was about 16 when I started a stamp company," Cuban told American Business Owner in 2010. "I started going to stamp shows and trade shows and just working a little bit harder than other people at these shows. I would trade up from one stamp to the next. I went to a show starting with only a quarter and started with buying a single stamp."

Success helped him gain confidence: "I left that show with $50 thinking, 'Hey, if I could do this, I could do anything.'"

And, as he told the United States Postal Service, "collecting stamps is an amazing way to start to understand business. Each stamp has its own level of scarcity, of demand, of price, and as a collector you have to make decisions on when to keep a stamp, trade or sell it, and when to invest in a new stamp for your collection."

The experience, overall, was a great education for him, he said: "I bought, sold and traded so many that the experience taught me as much about business as any class I have ever taken."

Cuban started looking for ways to earn money when he was 12 years old and his dad declined to buy him a pair of sneakers. "My dad said, 'Those shoes on your feet look like they're working pretty well. If you want a new pair of sneakers, you need a job and you can go buy them,'" he recalled during a 2014 episode of Bloomberg's "Masters in Business" podcast.

So he started selling trash bags around his neighborhood in Pittsburgh: "I would literally go door to door to door: 'Hi, does your family use garbage bags?' And who could say no? So that's where I learned to sell."

He's not the only billionaire who hatched business schemes as a kid.

Warren Buffett earned $5,000, the equivalent of about $53,000 today, before he turned 20, thanks to various business ventures. One of his most lucrative ideas involved setting up pinball machines in barber shops all over Washington, D.C., where his family lived at the time. A year later, Buffett sold the business, which he'd started with just $25, for over $1,000.

There's value in learning how to hustle from a young age, says Cuban. To succeed, "you've got to grind. It takes work. There are no shortcuts," he told Smart Hustle Magazine at SXSW in 2016. "There's gonna be things that upset you. There's gonna be failure and if you're not able to hustle, if you're not able to grind, you're not gonna be able to fight through those things."

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