There are many special properties of zero. For instance, you cannot divide by zero (or have zero as the denominator [bottom number] of a fraction). This is because, simply put, something cannot be divided by nothing. Thus, if some equation has a unit (usually a number) divided by zero, the answer is considered to be “undefined.” But it is possible to have zero in the numerator (top number) of a fraction; as long as it does not have zero in the denominator (called a legal fraction), it will always be equal to zero. Other special properties of zero include: Zero is considered an even number; any number ending in zero is considered an even number; when zero is added to a number, the sum is the original number; and when zero is subtracted from a number, the difference is the original number.