My brother is a peculiar liar. He always lies on Tuesdays, Thursdays and Saturdays. The rest of the days of the week, however, always tells the truth. On the other hand I am a clueless and I do not remember what day of the week I am. On Mondays, Tuesdays and Saturdays it is my turn to take the garbage to the street and on Wednesdays, Thursdays and Fridays it is my brother's turn. As I don't remember what day it is today, I decide to ask:
- What day is today?
- Today is Saturday, it's your turn to take out the trash.
- Are you sure?
- Sure, tomorrow will be Wednesday and it will be my turn.
What weekday is today?
Today has to be Thursday. The brother is clearly lying because he says that today is Saturday and tomorrow is Wednesday so it is one of the days he is lying: Tuesday, Thursday or Saturday. If it were Saturday, I would have told the truth "Today is Saturday, it's your turn to take out the garbage" so it has to be Tuesday or Thursday. If it were Tuesday I would have told the truth in the second sentence "Tomorrow will be Wednesday and it will be my turn" but we know that he is lying so today is thursday and it's up to him to take out the trash to the street.