Vayishlach - Sheini
"What Do You Want to Know THAT For?"
Yaakov prepared a gift for Eisav. There were hundreds of animals of various species, which he separated into herds. He sent each herd separately, with a different servant, each of whom was to tell Eisav that it was a gift from Yaakov, who was coming up behind them. He sent the herds ahead, then led his two camps to safety, across the river. After that, Yaakov was all alone, when he encountered Eisav's "guardian angel." They wrestled until dawn, when the angel saw he couldn't win. He touched Yaakov in the hip and dislocated it. The angel then asked to be released, since he had to attend to his morning duties. Yaakov told the angel that he would not release him until the angel blessed him. The angel blessed him by changing his name to Yisroel (Israel), meaning that he had prevailed in his struggles with both man and G-d. Yaakov asked the angel to reveal HIS name, but the angel said, "What do you want to know THAT for?" The angel blessed him and left.
You may be wondering how an angel gets the authority to rename Yaakov. Don't worry; G-d ratifies this name change Himself, in the fifth aliyah.