The <em>identity property</em>. When we're talking about addition and subtraction, the number 0 has the unique quality that it doesn't change whatever number it's added to or subtracting from. Though it's formally called the <em>identity element</em>, I like to think of it as a "mirror number;" it shows a number it's own reflection, confirming its "identity." If the number 53 wanted to know what number it was, it could add 0 to itself for a reminder (53 + 0 = 53, "Oh, that's right! I'm 53!")
