Day Two

And God said, “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.”  And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse.  And it was so.  And God called the expanse Heaven. And there was evening and there was morning, the second day.

Genesis 1:6-8  ESV

And God said...and it was so.

 This is showing us the power of God speaking.  He simply has to speak something into existence.   God spoke, the formless existence obeyed. 

God separated the waters so that there was water above and below the "expanse".  The water below is easy to understand: lakes, oceans, seas.   What is the water above?  The water in the clouds: snow and rain!

Another word for expanse could be "sky".