The water that falls from the sky today as rain may have once been part of the ocean. how can this be?

Water evaporates from the water in the ocean, river, creek, and any body of water ever. Then it is made into rain. When it rains it's like recycling. So it pretty much just evaporated from the ocean.

So the sun shines onto the ocean heating it up and water starts to evaporate. This makes clouds which you and I both see on a daily bases. Winds help to blow the clouds which might end up being the clouds above you right now. The clouds cool and condense which form water droplets, when these droplets get too heavy for the cloud to hold it falls creating rain. So this is why the rain that falls today may have been part of the ocean once. Hope this answers your question