Define tautology and contradiction. 

Ans: If a compound proposition comprises two atomic propositions as components, after that the truth table for the compound proposition consists of four entries. These four entries might be all T, may be all F, might be one T and three F and etc. There are in total 16 (2⁴) probabilities. The possibilities while all entries in the truth table is T, involves that the compound proposition is all time true. This is called tautology. 

Though, when all the entries are F, it implies that the proposition is never true. This situation is considered as contradiction.