The answer(this answer sucks so damn much and theres probably a better way to phrase it): "They were driving in opposite directions with their heads stuck out the windows to try to see in the fog and they smacked their heads into each other."
...Yeah, I know, craptacular, I'll choose better riddles from now on...anywho:
A man suddenly walks up to you and says "If you lie to me I will kill you with a knife., and if you tell me the truth I will kill you with a gun"
What must you say to stay alive?