It is tempting to think that some materials are always heavier than other materials. For example, someone might say that metal is heavier than air. But it depends on how much of each material there is ...

It makes sense that a balloon filled with a lightweight gas such as helium would float when surrounded by heavier air. But why does filling a huge balloon with hot air also make it float? This ...

A planet’s mass and volume are easy to determine. Because planets are nearly spheres, we can calculate volume from our knowledge of a planet’s radius, which we determine by measuring both the planet’s ...