When Kevin O'Leary took his teenage son, Trevor, to Europe, he flew first-class.

Trevor sat in the back, in economy.

While this might sound cruel and unfair, the "Shark Tank" investor wanted to teach his son a lesson about the value of money.

In his book, "Cold Hard Truth on Family, Kids and Money," the father of two explains, "One surefire way to nudge your teenagers into the workforce is by setting firm boundaries on what you will and will not pay for."

For him, that means drawing the line at luxury travel.

He writes:

At 16, my son is making the connection between money and personal freedom. I think that's the greatest gift I've ever given him: to help him see that connection. And I constantly reinforce it by doing Mean Dad things like making him sit in those crappy economy seats.

O'Leary worked hard to get to the point where he could afford a first-class ticket. He took a $10,000 loan from his mother and used it to build a $4 billion business.

"At this point in my life, I think I've earned the right to a more comfortable travel experience," he writes.

But his son hasn't.

O'Leary told his son that he could fly first class any time that he liked, so long as he paid for it with money that he earned himself.

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