Not Twins

A teacher enters the classroom and sees on the first row two students sitting next to each other, looking completely identical. She asks them if they are twins, but the students simultaneously reply that they are not. After checking in the records, the teacher furthermore discovers that the two children have the same mother and father. What is the explanation?

The students are three of a triplet.

My Son

After a car accident, the father dies and the boy arrives at the emergency room in the hospital. Upon entering the room however, the surgeon exclaims “This is my son, I can’t operate!” How is this possible? 

The surgeon is the mother of the boy.

Avoiding Bad Luck

You are walking alone on the sidewalk. There are no stars on the sky, no moonlight, all of the lamps on the street are broken, you don’t carry any source of light with you and there aren’t any cars or other people approaching. A silent black cat tries to cross your way, but you somehow spot it and turn around in order to avoid bad luck. How did you see the cat?

All of this happened during a day (which is cloudy).