This has always been a favorite of mine, and if you have seen it before please do not answer... If it has been posted before feel free to point that out...
A man and his son are turkey hunting together, and while crossing some railroad tracks, they are both struck by a train. The father is killed instantly, but the son survives with critical injuries.
When the boy arrives to the critical care unit, he is met by the doctor, who immediately looks at him and says "I cannot treat him, he is my son."
How is this possible?
As a hint, it there is a logical answer, and it is not a trick question.