They took him [Yosef], and threw him in the pit. The pit was empty; there was no water in it. (37:24)
Why does the verse say that the pit was empty, and also that there was no water in it? Isn't this redundant?
Rashi's answer is that “there was no water” implies that there WAS something else in it: snakes and scorpions.
The weaknesses of this explanation include: 1) there is no clear source that it was snakes and scorpions, not some other animal or object, 2) the presence of snakes and scorpions would be irrelevant to the rest of the story. So I want to take a different approach, which is closer to the pshat.
I think the “pit” under discussion was a cistern, a large hewn underground tank meant to hold water. (In many of the more obscure archeology sites across Israel you can see such cisterns.) Such a pit would fill with water during the winter rains, and the water would be preserved and used through the summer. As far as Yosef was concerned, there were three possible states for such a pit. It could be: 1) Mostly or totally full, in which case he would drown upon being thrown in, 2) Empty except for a little water remaining at the bottom, in which case he could live for a long time, drinking the water, and maybe be rescued, 3) Totally empty and dry, in which case he would soon die of thirst.
I think the repetition in the verse is needed to specify exactly what state the pit was in. “The pit was empty” rules out state 1, while “there was no water in it” rules out state 2. Thus, we know that the pit was in state 3, which also best fits the brothers' intentions regarding Yosef.
There is external evidence suggesting that the pit was in state 3. The brothers, who lived in Hebron, had gone to Shechem and from there to Dotan (near modern Jenin) to herd their sheep. What made them travel so far away from home? The answer is likely that it was a very dry year, and they had trouble finding plants for their flocks to eat. Thus, they went further and further north (north is wetter in Israel) until they were able to sufficiently graze their flocks. If the weather was so dry, then the cisterns would almost certainly be dry, as we have assumed.
(As is often the case, thanks to my havruta D.L., for pushing me on the question of this redundancy until I was forced to come up with an answer.)