Exodus 7:17

Turned to blood -This miracle would bear a certain resemblance to natural phenomena, and therefore be one which Pharaoh might see with amazement and dismay, yet without complete conviction. It is well known that before the rise the water of the Nile is green and unfit to drink. About the 25th of June it becomes clear, and then yellow, and gradually reddish like ochre; an effect due to the presence of microscopic cryptogams and infusoria. The supernatural character of the visitation was tested by the suddenness of the change, by its immediate connection with the words and act of Moses, and by its effects.