Simplify the following text for a Grade 4 native English-speaking person.
Original: Traditionally , the sequence of natural numbers started with 1 ( 0 was not even considered a number for the Ancient Greeks . ) However , in the 19th century , set theorists and other mathematicians started including 0 ( cardinality of the empty set , i.e. 0 elements , where 0 is thus the smallest cardinal number ) in the set of natural numbers .
Simplified: Some people say that 0 is a natural number , too .