In a modern slot machine, the odds of hitting a particular symbol or combination of symbols depends on how the virtual reel is set up. As we saw in the last section, each stop on the actual reel may correspond to more than one stop on the virtual reel. Simply put, the odds of hitting a particular image on the actual reel depend on how many virtual stops correspond to the actual stop. In a typical weighted slot machine, the top jackpot stop (the one with the highest-paying jackpot image) for each reel corresponds to only one virtual stop. This means that the chance of hitting the jackpot image on one reel is 1 in 64. If all of the reels are set up the same way, the chances of hitting the jackpot image on all three reels is 1 in 643, or 262,144. For machines with a bigger jackpot, the virtual reel may have many more stops. This decreases the odds of winning that jackpot considerably. The losing blank stops above and below the jackpot image may correspond to more virtual stops than other images. Consequently, a player is most likely to hit the blank stops right next to the winning stop. This creates the impression that they "just missed" the jackpot, which encourages them to keep gambling, even though the proximity of the actual stops is inconsequential. A machine's program is carefully designed and tested to achieve a certain payback percentage. The payback percentage is the percentage of the money that is put in that is eventually paid out to the player. With a payback percentage of 90, for example, the casino would take about 10 percent of all money put into the slot machine and give away the other 90 percent. With any payback percentage under a 100 (and they're all under 100), the casino wins over time. In most gambling jurisdictions, the law requires that payback percentages be above a certain level (usually somewhere around 75 percent). The payback percentage in most casino machines is much higher than the minimum -- often in the 90- to 97-percent range. Casinos don't want their machines to be a lot tighter than their competitors' machines or the players will take their business elsewhere. The odds for a particular slot machine are built into the program on the machine's computer chip. In most cases, the casino cannot change the odds on a machine without replacing this chip. Despite popular opinion, there is no way for the casino to instantly "tighten up" a machine. Machines don't loosen up on their own either. That is, they aren't more likely to pay the longer you play. Since the computer always pulls up new random numbers, you have exactly the same chance of hitting the jackpot every single time you pull the handle. The idea that a machine can be "ready to pay" is all in the player's head, at least in the standard system.