For some reason, there's a common rumor about sugar and gas that's been around for decades. Supposedly, if you pour sugar into someone's gas tank, you will disable the car. The sugar is supposed to react with the gasoline and turn into a semi-solid, gooey substance that totally clogs up the gas tank, the fuel lines, and so on.
It sounds great, especially if you have a grudge against someone. The problem with this rumor is that it simply isn't true. As it turns out, sugar doesn't dissolve in gasoline. Pouring sand into the gas tank would have about the same effect as pouring in sugar. The sand or sugar might clog up the filter, and that could disable the car, but it's not a sure thing.
So what do you do if you really do want to disable someone's car? Pour in some water. Gasoline floats on top of water, so if you pour in several cups of water, the fuel pump will fill the fuel lines with water instead of gasoline and the car would have some major problems.