Fossil fuels are substances that can be burned to produce energy. The burning of fossil fuels releases certain gases into the air. These gases combine with other chemicals in the air to form substances called acids. When trees on the top of mountains are exposed to clouds that contain these acids, the acids remove substances from the trees that help the trees fight against harmful insects.

In an experiment examining the effects tai chi on arthritis pain, callahan (2010) selected a large sample of individuals with doctor-diagnosed arthritis. half of the participants immediately began a tai chi course and the other half (the control group) waited 8 weeks before beginning. at the end of 8 weeks, the individuals who had experienced tai chi had less arthritis pain that those who had not participated in the course.