This 20-answer worksheet is designed to accompany Episode 7, The Clean Room, of the documentary television series Cosmos: A Spacetime Odyssey hosted by Neil deGrasse Tyson and follows up Cosmos: A Personal Voyage presented by Carl Sagan. This episode is centered around how science, in particular the work of Clair Patterson in the middle of the 20th century, has been able to determine the age of the Earth. Tyson first describes how the Earth was formed from the coalescence of matter some millions of years after the formation of the Sun, and while scientists can examine the formations in rock stratum to date some geological events, these can only trace back millions of years. Instead, scientists have used the debris from meteor impacts, such as the Meteor Crater in Arizona, knowing that the material from such meteors coming from the asteroid belt would have been made at the same time as the Earth. Tyson then outlines the work Patterson did as a graduate under his adviser Harrison Brown to provide an accurate count of lead in zircon particles from Meteor Crater, and to work with similar results being collected by George Tilton on uranium counts; with the established half-life of uranium's radioactive decay to lead, this would be used to estimate the age of the Earth. Patterson found that his results were contaminated by lead from the ambient environment, compared to Tilton's results, and required the construction of the first ultra-high cleanroom to remove all traces of environmental lead. With these clean results, Patterson was able to estimate the age of the Earth to 4.5 billion years. Tyson goes on to explain that Patterson's work in performing lead-free experiments directed him to investigate the sources for lead. Tyson notes how lead does not naturally occur at Earth's surface but has been readily mined by humans (including the Roman Empire), and that lead is poisonous to humans. Patterson examined the levels of lead in the common environment and in deeper parts of the oceans and Antarctic ice, showing that lead had only been brought to the surface in recent times. He would discover that the higher levels of lead were from the use of tetraethyllead in leaded gasoline, despite long-established claims by Robert A. Kehoe and others that this chemical was safe. Patterson would continue to campaign against the use of lead, ultimately resulting in government-mandated restrictions on the use of lead. Tyson ends by noting that similar work by scientists continues to be used to help alert mankind to other fateful issues that can be identified by the study of nature. Applies to science, astronomy, physics, history, chemistry, nuclear physics, astrophysics, geology, organic chemistry and environmental quality.