expression
off the top of one's head; from thin air
Used when saying or doing something without preparation, evidence, or reference; often implies improvisation or making things up on the spot.
彼は空でスピーチをした。
He gave a speech off the top of his head.
そんな話は空で言っているだけだ。
He's just saying that out of thin air.
expression
from memory; by heart
Used when reciting, singing, or reproducing something entirely from memory without looking at notes or text.
彼女は長い詩を空で暗唱した。
She recited the long poem from memory.