This article describes the differences of good ozone and bad ozone. Good ozone, which is found in the stratosphere, protects people and other living things from the bad things UV can do, such skin cancer, cataracts, and other problems. However, lower in the atmosphere, at the top of the troposphere (around 12 miles up), ozone acts like a greenhouse gas, trapping heat from Earth and preventing it from escaping into space. Scientists use an instrument called spectrometer to understand the complicated ozone chemistry. This article describes how a spectrometer can be made in the classroom. (Contains 1 footnote.)