Brahman put a straw before him, saying: 'Take it up.' He went towards it with all his might, but he could not take it up. Then he returned thence and said: 'I could not find out what sprite this is.'

In plain words: Brahman puts a single straw in front of Vayu and tells him to lift it. Vayu tries with all his strength but cannot move it, so he goes back and admits he could not find out who this being is.