I assume you mean trees are mostly made from air and not trees are mostly gaseous. Most of trees are carbon, which they get from carbon dioxide in the air, but they transform it, using energy from the sun. Equivalently you could say they’re mostly made from sunlight, which is obviously wrong, but equally accurate.
The statement that humans are mostly water, on the other hand, is actually correct. It is water in the form of water from water.
Trees are mostly made of air.
I assume you mean trees are mostly made from air and not trees are mostly gaseous. Most of trees are carbon, which they get from carbon dioxide in the air, but they transform it, using energy from the sun. Equivalently you could say they’re mostly made from sunlight, which is obviously wrong, but equally accurate.
The statement that humans are mostly water, on the other hand, is actually correct. It is water in the form of water from water.