This Shockwave tutorial for Grades 5-8 explores properties of gases in an interactive, game-like environment. It was designed to help students build concepts about mass, volume, and density of gases. Activity 1 dispels the myth that “”gas is weightless”” (a commonly-held misconception among secondary students). Activity 2 demonstrates density as students place helium, air, water, and oil in a virtual cylinder. Activity 3 illustrates what happens at the molecular level as gases are heated or cooled or undergo pressure changes. Editor’s Note: This item is part of NOVA’s “”Balloon Race Around the World”” web site. SEE RELATED ITEMS for a link to a teacher’s guide and additional interactive resources. Teachers’ Domain is an NSF-funded pathway of the National Science Digital Library (NSDL). It is a growing collection of more than 1,000 free educational resources compiled by researchers and experienced teachers to promote the use of digital resources in the classroom.