What are common English brain teasers?

Simple one for English language students

A man and his son are in a car accident and taken to the hospital.

The surgeon enters the operating room, looks down and says, "I can't operate on this patient. He's my son!"

How is this possible?

The son was the surgeon's grandson, the man was the surgeon's son.

Some others:

Here's one, based on logic:

You have 2 identical doors before you. You must go through one of them. Behind one door is freedom, behind the other is imprisonment for life. You do not know which is which. Each door is guarded by a doorkeeper. You know that the keeper of the door to freedom never lies; he always tells the truth. You know that the keeper of the door to imprisonment always lies; he never tells the truth. You can ask one of the doorkeepers one question. Then you must make your choice. What do you do?

Here's another:

When is a door not a door?

This one is a little tougher than meets the eye:

If 1.5 (one and a half) kids can make 1.5 kites in 1.5 days, how many kites can 6 kids make in 6 days?

Can you name an English word that contains all of the vowels in order, including y?

And Another Logic Brainteaser:

A one-armed surgeon needs to operate on three patients but he only has two individual surgical gloves and he can't get any more. How can he operate on the three patients in turn without infecting the patients or himself?

[Look in Discussion section for answers]