God changes Sarah's name to the plural form because she will be princess to many now that she is having her own child. When Sarah finds out that she will have her own son, she laughs because she is almost 90 years old. But nothing is impossible for God and she does get pregnant and have a son. Meanwhile, Abraham is concerned for Ishmael because he thought the promises would all be fulfilled through him. God eases his concern by telling him that Ishmael also will become a great nation. The promise will just not come through him.
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