Acid rain forms when harmful gases in the air mix with falling rain, lowering its pH and making it damaging to plants and soil. This experiment puts that harm to a direct test: five pots of sunflower seeds are watered with solutions ranging from pH 6 down to pH 2, using sulfuric acid to hit each target level. At pH 6 and pH 5, the plants grow normally. Drop to pH 4, and growth slows down sharply. At pH 3 and pH 2, the seeds never sprout at all.