Two sets have the same cardinality if we can pair up their elements—if we can join two elements, one from each set. The set formula_14 and the set formula_16 have the same cardinality. E.g, we could pair "apple" with "sun", and "orange" with "moon". The order does not matter. It is possible to pair all the elements, and none is left out. But the set formula_17 and the set formula_18 have different cardinality. 