Acid rain is rain that has a larger amount of acid in it than what is normal.
Acid rain is caused by smoke and gases that are given off by factories, and vehicles that run on fossil fuels. When these fuels are burned to produce energy, the sulphur that is present in the fuel combines with oxygen, and becomes sulphur dioxide.
Some of the nitrogen in the air becomes nitrogen oxide. These pollutants go into the atmosphere, and become acid. Sulphur dioxide and nitrogen oxide are also produced when coal is burnt for fuel.
Acid rain destroys trees, harms wildlife and eats away the stone work in buildings. It also gives us asthma, and coughs!