We count things using numbers.
For example, this is one butterfly and these are 4 butterflies.
We start counting things from 1. The numbers 1, 2, 3, 4, ... are called counting numbers or natural numbers.
How do we count something that was there and is no more?
For example, if there were 4 puppies and now there’s none. How do we show none or nothing?
We use Zero (0) to show nothing.
The counting numbers or natural numbers along with zero form whole numbers.
We use the digits 0 to 9 to form all the other numbers. Using these 10 digits we can form infinite numbers.
For example, 121; 34987; 2987633; 459227904; ...
This number system using 10 digits is called Decimal Number System.
There are many ways in which we can classify numbers.
Even Numbers 
Odd Numbers 
Numbers that end with a 0, 2, 4, 6 or 8 are called even numbers. Example, 14, 202, 500, 8146, 99718 Even numbers are evenly divisible by 2. Example, 244 ÷ 2 = 122 
Numbers that end with 1, 3, 5, 7 or 9 are called odd numbers. Example, 11, 213, 6005, 2097, 36739 Odd numbers are not evenly divisible by 2. They always leave a remainder. Example, 245 ÷ 2 gives 122 as quotient and 1 as remainder. 
Prime Numbers 
Composite Numbers 
Numbers that are divisible by only 1 and themselves are called Prime numbers. A prime number has only 2 factors, 1 and itself. Example: 1, 2, 3, 5, 7, 11, 13, 17, 19,23, 29, ... 
Numbers that are divisible by numbers other than 1 and themselves are called composite numbers. A composite number has more than 2 factors. Example: 4, 6, 8,9, 12, 14, 15, 16, 200, 1122 
Fun Facts
