Q: A boy is brought into the hospital needing urgent surgery and the doctor takes one look at him and says, I can't operate on him. Why?
A: Because the doctor is his mother.
There's a whole series like these - based on assumptions about people in certain professions being more likely to be male. Another one is:
Q: A policeman is working past a room. The window is too high to see in. The person hears "no John, don't", and then a gunshot. He rushes inside and sees a dead body on the floor with a gun beside him. Also in the room are a doctor, a lawyer and a priest. Without asking any questions, he immediately arrests the priest. Why?
A: Because the priest is the only male in the room.
A few other brain teasers and riddles:
Q: A group of explorers in the Arctic come across a naked man frozen in a block of ice. The leader of the expedition immediately says "That's Adam". He was right, but how did he know.
A: Because he had no belly button (Adam is the only human who (allegedly) was not born of a woman - so he would have no belly button).
Q: If a plane crashes on the border between Canada and the US, where do they bury the survivors?
A: You don't bury survivors.
Q: A man is dead in a cabin in the woods, how did he die?
A: A plane crash (it was a plane cabin - brain teaser relies on people assuming it's a log cabin).
Q: Romeo comes home drunk one night and the next day they find Juliet dead in a pool of water, how did she die?
A: He knocked over her bowl - Juliet was a goldfish.
Q: Your in a dark room with only one match. There's a candle and a lantern. What do you light first?
A: The match.
