We all know that friends come and go as we make our way through life's various stages.

Some of us, however, are still BFFs with the person who was placed next to us in our Junior Infants class, and, as a result, they have seen us through countless phases over the last two decades.

And by result, we have seen them through the same, meaning we have the skinny on their every move.

So without further ado, here are 15 things you could only know about a person you go this far back with…

1. Their unique childhood habits.

Growing up with a person means you're subject to private knowledge about some of their most peculiar childhood habits – habits they'll convince you they've grown out of it, but you know better.

"Remember the way you used to do the Cha Cha slide at every disco? Pretty sure you were doing it in the pub last night."

2. That odd irrational crush they had on that one person

You don't make it this far in life without having at least one crush you cringe over now.

Lucky for your best mate, you remember in excruciating detail exactly how much she was obsessed with that random corner shop cashier guy.

"Remember when you used to put on a full face of makeup when your Mam would ask you to run down to the shop for a pint of Avonmore, just in case yer man was in there?"

3. The points they got in their Leaving Cert.

Nowadays, as long as your friends are content in their college and career choices, you pay little attention to the ins and outs of their results, but back in the day, you knew your friends grades almost as well as your own.

"You're bound to get a B on that paper, but I'll scrape a C. I'll beat you in English though because I read the book and you didn't."

4. The name of their very first crush.

While you may think you know the ins and outs of your friends' relationships right now, current intelligence has nothing on the information you had on your mate's first crush.

"Remember the socks he always wore on non-uniform days? And what about that clicky pen he only used for History?"

5. The appalling outfit they wore to death.

Reminding your friends of their poor fashion choices is the sole reserve of people who have been friends since primary school.

"Remember your Susst jeans that you used match with that Gap hoody that didn't actually fit you? And that bakerboy cap?"

6. That 'thing' that happened 'that' time.

Everyone has a deep dark secret – a moment they would give anything to erase from their personal history – and you know this friend's one all too well.

"Here, remember the time you decided to prank call the principal….?!"

7. Their siblings' worst habits.

When you grow up with someone, you know their siblings' foibles as well as they did, and you were well within your rights to call them on it.

"Remember the way your brother used chew really loudly with his mouth open? That was really gross."

8. Their middle names

Whether she plumped for the classic saintly moniker on her Confirmation Day, or went for something a little more exotic, you know exactly where her middle names came from.

"I know I agreed that Britney was a top-notch choice back in 2000, but I was wrong."

9.  Exactly how much they earn

Only your true bestie knows exactly how much you pull in each month, and doesn't judge you for it either way. 

Oh, and you still go even-Steven splits on everything.

"Gal you get paid bi-weekly and I get paid monthly, would you mind grabbing us those flights and I'll get you back at the end of the month?"

10. The voice they use when they're nervous.

Some things never change, and your mate's habit of going ultrasonic when faced with an awkward situation is one of them.

"I came over here to rescue you cos your voice is shattering glass right now."

11. The 'incident' their parents still don't know about.

There are some moments from your teenage years you were lucky to make it out of alive, and only you and your primary school BFF know the true extent of it.

"Imagine your mam found out what happened on Hallowe'en night 2002!"

12. The 'incident' they think YOU don't even know about.

Everyone has secrets; unfortunately for your primary school BFF, nothing gets by you, and that thing they think you don't know about, you do.

"So, whatever happened to those singing lessons you used to attend…?"

13. The way they take their tea.

Yes, it's a given you should know how most of your friends take their tea, but when it comes to your primary school BFF, you're a certifiable pro.

"Medium strength, Avonmore SuperMilk, one spoon of sugar and three quick swirls of the spoon."

14. The thing they're secretly so proud of.

Everyone has moments they're proud of and that's fine, but your mate harbours a secret sense of pride over the most random things, and only you know about it.

"So… do you still watch the recording you have of that time Ant and Dec read out your letter on SMTV?"

15. The things that will most definitely make them cry.

Some things are a given, but only you know the bizarre things that will set your friend off.

"Now, don't look, but there is a little old man reading a paper on a bench over there. Jaysus, don't look, you'll bawl."