This is a trick question! There isn't a single "lightest thing" on Earth because "lightness" can be measured in different ways:

* Weight: The lightest thing by weight would be something with very little mass. A single atom of hydrogen would be incredibly light.

* Density: The lightest thing by density would be something that takes up a lot of space but has very little mass. A balloon filled with helium would be very light compared to its volume.

So, it depends on what you mean by "lightest"!