What are "Chartumim;" and what are "Lateihem"?
Refer to 7:11:2:1; 7:22:0.1:1; 7:22:2.3:1; and 7:22:2.4.
Why did Pharaoh not ask his magicians to remove the frogs?
Oznayim la'Torah: Because they were not careful about the three things that protect a person against witchcraft, 1 in which case they were not able to remove the witchcraft from others either.
Chulin 105b: These are; not to clean oneself after defecating with a piece of clay taken from the floor of the bathroom, not to kill lice while they are on a garment, and not to eat a vegetable from a bunch tied by the gardener.
The magicians made a point of producing blood and frogs. Why did they make no effort to remove any of the plagues?
Oznayim la'Torah: Because they were wise about how to do harm, but not to do good by removing it.
