The text is a play on the idiom 'feet firmly planted on the ground', which means being realistic and practical. However, by saying 'in the air', it creates a humorous image and a contradiction, as one cannot have their feet 'firmly planted' in the air, which is not a solid surface. This juxtaposition creates a humorous effect, but it does not rely on different meanings of the same word or similar-sounding words.
Both of his feet are firmly planted in the air .