שמות, פרק ז׳, פסוק י״ט

The first plague to strike Egypt transforms from a localized demonstration of divine power into a comprehensive national punishment. Initially, Moses was told to strike only the river to prove God's existence to Pharaoh. However, the command quickly expands to target all the water sources in the land, serving as a severe penalty designed to deprive the Egyptians of drinking water [מלבי״ם].

God instructs Moses to hand the execution of this plague over to his brother, Aaron. The primary approach among commentators is that this transfer stems from a deep sense of gratitude. Because the river had protected Moses when he was hidden there as a baby, it was considered improper for him to strike it [רש״י, מזרחי, גור אריה, ברכת אשר]. Other perspectives suggest that Moses could not use his staff to strike the water because it had previously turned into a snake, a symbol of accusation [משכיל לדוד]. Alternatively, Aaron was specifically chosen due to his lifelong dedication to fostering peace and family growth among the Israelites. This served as a direct counterweight to the harsh Egyptian decrees that had prevented Israelite women from immersing in water to purify themselves [אלשיך].

To enact the plague, Aaron was not required to physically travel throughout Egypt. The accepted view is that after striking the river, Aaron remained in his place and waved his staff toward all directions. This symbolic gesture ensured the plague would apply to all waters simultaneously, proving to Pharaoh that this was not a random natural occurrence, but a deliberate, calculated action by God [אבן עזרא, רש״ר הירש, קאסוטו, שטיינזלץ].

The devastation targeted all fresh, drinkable water [הכתב והקבלה]. This included a wide variety of sources: natural flowing rivers, artificial canals and pools dug by humans to irrigate fields, and stagnant bodies of standing water, such as accumulated rainwater. It even reached springs, wells, and the bathwater located inside private homes.

The absolute reach of the plague culminated in its effect on wood and stone. Most commentators explain that this refers to water that had already been drawn and stored inside the Egyptians' homes in wooden and stone vessels or large troughs [רש״י, אבן עזרא, שפתי חכמים, פרדס יוסף]. However, some identify a sharp irony directed at Egyptian culture. The Egyptians typically drew muddy river water into special wooden and stone containers designed to filter and purify it. The plague demonstrated that even these advanced purifying vessels were completely useless against God's decree [הכתב והקבלה]. Another perspective views the wood and stone as a subtle reference to Egyptian idols. The priests would wash their wooden and stone statues early every morning, and even this specific water turned to blood, serving as a profound mockery of the Egyptian gods [קאסוטו, ש״ך]. Conversely, a different approach suggests that the water inside the household vessels did not literally transform into blood. Instead, the properties of the water changed, permanently staining the wooden and stone containers red [העמק דבר].