What did God make on the second day?

Answer: The expanse of sky

God made an expanse to separate the waters above from the waters below, and He called the expanse "sky."

“And God said, “Let there be an expanse between the waters, to separate the waters from the waters.” So God made the expanse and separated the waters beneath it from the waters above. And it was so. God called the expanse “sky.” And there was evening, and there was morning—the second day.”— Genesis 1:6-8 (BSB)