Rashi on Genesis 18:15/רש"י על בראשית י״ח:ט״ו

1כי יראה וגו' כי צחקת FOR SHE WAS AFRAID… BUT THOU DIDST LAUGH — The first כי is used in the sense of “because”, giving a reason for the former statement—Sarah denied … because she was afraid; the second כי is used in the meaning of “but” — “and He said, ‘It is not as you say that you did not laugh, but thou didst laugh’.” For our Rabbis say (Rosh Hashanah 3a) that the word כי has four meanings: “if”, “perhaps”, “but”, “because”.