For the morbidly curious -- ``Note how the trade executions ping pong from bid to ask. As if someone is buying at the offer, then 10 ms later selling at the bid, and so forth. It turns out, the gray shading you see in the first chart of EXC is from this alternating between buying and selling. That's right, almost all these trades alternate between buying at the offer and selling at the bid, which means losing the difference in price. In the case of EXC, that means losing about 15 cents on every pair of trades. Do that 40 times a second, 2400 times a minute, and you now have a system that's very efficient at burning money.''

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