Excel: Entering leading zeroes

Excel is often used as a database to hold information and not necessarily calculate numbers. One example is a mailing list. On campus, you only need to include the four-digit extension for the postal code. My mailing address would look like this:

Ranelle Maltas

104 ARCH

CC 0104

Here it is in Excel:

But how did I get the leading zero to stay? Usually when you type a number, such as 0104, it is formatted as a number, becomes right-aligned and drops the zero. You can format the cell as text instead, but that doesn't always help. What you need to do is type an apostrophe (') first. Don't worry, it will disappear like the image above. Here it is again in edit mode: