Explain why water used by plants and animals today has been around for millions of years.

The water that exists on earth is constantly undergoing the process of the water cycle. The water content is balanced due to this cycle. There is abundant water existing in three different forms namely solid, liquid and gas.  

According to the water cycle, the water in the oceans gets evaporated and rises up due to the evaporation process. This water vapors then goes into the atmosphere and when condenses forms clouds and precipitates back on earth in the form of rainfall, snowfall, and dew. This water flows over the inland areas and adds to the streams, lakes, and rivers. Some water percolates down through the rocks and mixes with the groundwater. These waters eventually mixes with the seas and oceans. Again, some parts of water are absorbed by the plants and trees and the remaining is released by the evapo-transpiration method.

So the water basically is circulating from the land to the sea and to the atmosphere and has been there for a long period of time, for about millions of years. This water is still being used by the plants as well as animals now.