Sentence:  For any <WSD>infinite</WSD> set, there is a 1-1 correspondence between it and at least one of its proper subsets. For example, there is a 1-1 correspondence between the set of natural numbers and the set of squares of natural numbers, which is a proper subset of the set of natural numbers. 
Meaning of the word marked with <WSD> tags:  Having infinitely many elements.