One day, Pip asks his parents: "What day is it today?"  His mother says: "Today is Monday."  His father says: "Today is Tuesday."  Which of the following is true? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
"Today is Monday" is not the opposite of "Today is Tuesday".i.e. they can both be false.  "Today is Monday" is the direct opposite of "Today is not Monday". One must be true and the other must be false. 