Exodus 7:19- "Blood will be everywhere in Egypt, even in the wooden buckets and stone jars."

Water that was needed for daily use was kept in these receptacles, even as was done where Rebekah lived (Gen.24:15).

Some of the stone jars had filtering pots of white sand through which the water was poured into the jars.

There were also stone reservoirs on street corners and other places which were for common usage, much like public drinking fountains are today.

Even though these receptacles were not connected to the Nile, God turned the water in them also into blood, which eliminates the possibility that the Nile turned red because of the reflection of the setting sun or some other natural biological problem- as some have suggested.