Why did Sarah laugh when she heard that she'll have a child despite her age (18:12) when she had just became a Nidah (Rashi 18:8) after not being a Nidah for a long time (18:11)? Shouldn't she realise that she can have children if she's a Nidah?
(Da'as Z'kenim Mi'Ba'alei Tosafos says that 18:11 means that she wasn't a Nidah for a long time, but now she was. Otherwise it'd contradict Rashi.)