Actually it is a good experiment. Not all gasoline is burned. A lot of it is spilled or leaked at gas stations. I just did an experiment on whether or not radish seeds would germinate in gasoline fumes. It turns out the seeds can germinate, but the plants become rather sickly from the fumes right away. Some turned a brownish-red color and the growth in all the plants were stunted. Be careful using the gasoline though. The fumes are stronger than you might think. You can't really tell when you are pumping gas because most gas stations have the black pollution guard on the pump. The fumes are pollution, which is why the EPA required those black pollution guards on all the pumps. It's a fun experiment.