Genesis 37:24

Since the verse says "the pit was empty", did I not already know that it didn't contain water? So what is the verse trying to communicate by pointing out that it does not contain water - namely, that while it doesn't contain water, there is something that it DOES contain: snakes and scorpions.

and took and threw him into the pit; but the pit was empty, no water was therein, but serpents and scorpions were in it.

אין בו מים. If the pit had been full of water the brothers would not have thrown him into the pit, as they then would have been guilty of drowning him, i.e. killing him with their own hands, as it were. They had said themselves “let not our hands be on him.”

ויקחהו, there is a letter ו missing in this word, making it appear as if only one brother took him. According to Bereshit Rabbah 84,16 this is an allusion to who the brother was who seized Joseph and threw him into the pit. It was Shimon, which was the reason why later on Joseph locked up Shimon as a hostage (42,24 ויקח מאתם את שמעון, he took Shimon from them.)

אין בו מים. If there had been water in the pit, Joseph would have drowned and throwing him into the pit would have been outright murder. The reason why the Torah mentions that there was no water in the pit, having already said that the pit was empty, והבור ריק, could be that there was sticky mud inside it, as for instance in Jeremiah 38,6 where the King’s servants threw the prophet into טיט, meaning a slime pit. That pit is also described as devoid of water, although Jeremiah is described as sinking into the mud. However, it did not cause his death. The allegorical explanation of the sequence of the words in our verse is well known, i.e. though water, one lethal ingredient, was not present in that pit, other potentially worse dangers such as scorpions, etc., were. If this explanation corresponds to the facts, throwing Joseph into such a pit was no better than killing him. The brothers had no way of knowing that G’d would save him by a miracle, i.e. that the scorpions and snakes would not attack him.

והבור רק אי בו מים, “the pit was empty, not containing any water.” Rashi comments that this is to tell us that while there was no water in the pit in which he could have drowned, there were other hazardous inhabitants in that pit such as snakes and scorpions. (based on B’reshit Rabbah 84,16.) This interpretation is based on the principle that when two negatives follow one another this indicates that we are being told something positive, even if not beneficial. It would have sufficed to describe the pit as simply: “empty.” If you were to ask why did it have to refer to the presence of scorpions? What hint is there of that? Maybe there were merely stones inside the pit? The expression רק for empty, instead of ריק means that it was empty on occasion but filled with water at other times. Scorpions take refuge there when it has been emptied from water.