Day Sixty-Three

I was with my aunt at the local beach when she asked me what was between the ocean and the sky. I looked at the horizon, thought about this a bit and thought there was nothing in between. She said the answer was a "to", a word for "and", which has the same sound as the word for a door. It was supposed to be a riddle but her answer left me unsatisfied. I remember this when I see the ocean and the sky.