High-frequency traders make money in a vacuum, grabbing for pennies that appear and disappear like the virtual particles of quantum field theory. Their goal is to end each trading day “flat”—out of the market, their profits safely in the bank. Depending on their model, they can do well winning as little as 55 percent of their trades. They are continuously testing prices, looking for patterns and trends or the chance to buy something in one place for $1 and sell it somewhere else for $1.01, or $1.001. Sometimes they aren’t even looking to make money on the trade itself. Under the “maker-taker” model, some exchanges offer tiny incentive payments, or rebates, for posting a quote (to buy or sell a stock) that results in a trade. The exchange charges the other side in the trade, the taker, a slightly higher fee and collects the difference. So an algo can buy a stock, earn a rebate, then sell the stock and earn a rebate for that too.