Volcanic eruptions can be violent enough to inject sulfur dioxide into the stratosphere: the layer of the atmosphere which contains the ozone layer. Sulfur dioxide is a gas, but once it is in the stratosphere various chemical reactions convert it into tiny particles. These particles can alter the Earth's climate by reflecting sunlight. In this paper we describe how we used a satellite instrument called the Microwave Limb Sounder to observe volcanic sulfur dioxide in the stratosphere.