Addition is an operation in which two numbers, called the addends, produce a third number called the sum. Natural numbers are added by starting with the first addend and counting as many more numbers as the second addend. For example, for 2 + 4, you would think 2, 3 (as the first number after 2), 4 (as the second number), 5 (as the third number), and 6 (as the fourth number), thus the numbers add up to 6. Not all numbers are added in the same way as natural numbers.