en_tn/gen/17/17.md

said in his heart

"thought to himself" or "said to himself silently"

Can a child be born to a man who is a hundred years old?

Abraham used this rhetorical question because he did not believe that this could happen. Alternate translation: "Surely a man who is a hundred years old cannot father a child!" (See: rc://en/ta/man/translate/figs-rquestion)

How can Sarah, who is ninety years old, bear a son?

Again Abraham used a rhetorical question because he did not believe this could happen. The phrase "who is ninety years old" tells why Abraham did not believe that Sarah could bear a son. Alternate translation: "Sarah is ninety years old. Could she bear a son?" or "Sarah is ninety years old. Surely she could not bear a son!" (See: rc://en/ta/man/translate/figs-rquestion and rc://en/ta/man/translate/figs-distinguish)