I was reading the Wikipedia entry for Hercules when I came across a reference to Heteropaternal Superfecundation. Apparently it is possible for a women to give birth to fraternal half-sibling twins. Obviously it is quite rare, but still pretty interesting.
The entry isn't very long so I recommend checking it out. Give yourself a factoid to boggle some minds at parties.
"There are more things in heaven and earth, Horatio,
Then dreamt about in your philosophy."
-Hamlet Act 1, Scene 5