How Stuff Works
"Information is moving so quickly and stocks are trading all day long," Maria says. "Let me give you a quick analogy. You're at home, you call your broker at [UBS] PaineWebber and you say ... 'I want to buy IBM but I'm not paying any more than $70 a share.' He's got to get over to that post before that stock changes in price. ... He's physically running over because if somebody gets in front of you and they want to buy a million shares, the next thing you know, it's $72 [per share]. Then he's got to call you and say, 'Guess what, Oprah? I couldn't buy it at $70. You paid $75.' And you're going to say, 'You're fired.'"