Adam Sandler's Big Daddy is about a so-called "loser" who lives in a crappy apartment and spends his time pissing the day away and working a toll booth on the side. Except that he's secretly a millionaire, or at least very well off. How else could he afford a multi-level loft in New York City?
In the shopping scene, we see Sonny shopping with his adopted son, Julian. Sonny throws a can of soup onto the ground claiming that dented cans are half price, and that "Microsoft went down 3 points" so he has to save some money.
Sonny is clearly a shareholder in Microsoft. Otherwise, why would he care about the share prices of one company? He wants to save money because Microsoft's share price is down which therefore would effect any dividend he receives from the shares in the company that he owns.
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Who bought these shares? It was clearly not Sonny, shown to be a worthless loser until halfway through the movie. I submit that it was his father who bought the shares for him when he was younger, so that Sonny has a lifelong source of income - share dividends - no matter what he does.
We've established that Sonny owns shares in Microsoft bought for him by his father - but how big of a stake does he own? It is impossible to know exactly how much, but we do know that he lives in a loft in New York City while working part time as a toll booth operator, so we can conclude that Sonny owns enough of a stake in Microsoft that he will never have to work a day in his life.