Rashi wrote: "In the wood and in the stones – water that is in wooden and stone vessels." We need to understand why the Torah specifically highlighted the wooden and stone vessels. I found in the Chumash of the Chida, where he brings from the commentary of Rabbi Elazar of Worms who wrote the following: "In the wood and in the stones, but not in metal vessels. This is why iron is placed in water during the equinox because the harmful entity is not found in metal vessels. Therefore, Pharaoh did not request metal vessels during the plague of blood, as he would drink from metal vessels and they did not turn into blood." This means that the Torah was precise in saying 'in the wood and in the stones' because the plague only affected these vessels and not metal ones.

In the Torah commentaries of the Tosafists (Tosafot HaShalem, Gliss, verse 19, letter vav) it is brought in the name of Rabbi Shimon that the wood and stones mentioned here refer to idols and statues. He added that the gematria (numerical value) of 'in the wood' is equivalent to 'in the statues' [212]. This explains why the Torah emphasized this, to convey that even their gods were affected by this plague.

My uncle, the esteemed Rabbi Zalman Sorotzkin ztz"l, wrote in his book "Oznayim LaTorah": "In the wood and in the stones – see in the Maharsha that blood was oozing from the wood and stones themselves, this is the blood of the children of Israel, who were placed in the buildings in place of wood and stones." This fits well with our explanation that the primary reason for the plagues upon the waters of Egypt was due to the spilling of the blood of Israel. It was fitting and appropriate that at that time, the spilled blood of the children of Israel should ooze from the buildings where they were placed instead of wood and stones.