A (dry) sponge is much lighter than water, but it might not float; when you put it in water, the holes fill up with water, and the question becomes whether the

A (dry) sponge is much lighter than water, but it might not float; when you put it in water, the holes fill up with water, and the question becomes whether the rest of the sponge is light enough to float. It's the same thing here. When they say it's lighter than air, they're counting the whole structure's volume, with all its holes in it. When you put it in air, and the holes fill up with air, the question then becomes whether the rest of it is lighter than air on its own. (Probably not)