What part of Ymir's body became the sky?

What part of Ymir’s body became the sky? His skull became the sky.

In the Norse creation account, Odin and his brothers Vili and Vé killed the primeval giant Ymir and transformed his body into the world. They raised his skull above the earth to form the heavens. Four dwarfs—Norðri, Suðri, Austri, and Vestri—were placed beneath it to support the sky at its corners.

The same cosmic dismemberment assigned different parts of Ymir to different features: his flesh became the earth, his blood became the sea, his bones became mountains, his hair became trees, and his brains became clouds. His eyebrows were used to enclose Midgard, the human world.

This image appears in both the Prose Edda and the Poetic Edda. A passage in Vafþrúðnismál specifically describes the sky as coming from the frost giant’s skull. Readers sometimes confuse the skull with the bones, but the bones became the mountains; the skull alone became the heavens.