DD1 is 3.5 and she knows that we get mail and boxes delivered, etc. But I didn't know that she *really* understood!
Yesterday, she drew a picture for her cousin then asked me to put it in the mailbox for her. I asked her why, and she said so the mail man can take it to her cousin! LOL So I explained to her we have to put it in a envelope, address it, and put a stamp on it. She was so excited to help me "stamp it". LOL Now her first piece of mail is on it's way to her cousin via the United States Postal Service. Gotta love snail mail!