As a teacher, I have had variants of this conversation many times. The specific details, however, are fictional, for this changes, somewhat, each time it happens.

• Student: Guess what? It’s my birthday!
• Me: Congratulations! How old are you?
• Student: I’m seventeen!
• Me: Well, happy 18th birthday, then!
• Student: Huh?
• Me: Look, on that one day, 17 years ago, when you were born, that was your birthday. That day has a better claim on being your birthday than any other day, because it’s the day you were born. That was your first birthday. But you weren’t one year old yet. You turned one year old a year later, on your next birthday . . . your second birthday. A year later, on your third birthday, you turned two years old. Do I need to continue?
• Student: So I’m 18? I can buy cigarettes without a fake ID, and vote, and stuff?
• Me: No, not for another year, because you’re only 17 years old — but you have had 18 birthdays. Say, here come some of your friends. Use this bit yourself, if you want to, and have fun with it.
• Student, to other students: Hey, guys, it’s my birthday! I’m 18 today!

…At least I try. Also, sometimes, the educational outcome is better than in this fictionalized example.

[Image source: http://www.decorationnako.tk/birthday-cake/]