Alex was still sad the next day, so I had a plan...

"Alex!, Tali!, I need you for a moment"
"What is it Libby?"
"Well Alex, I need you guys to do something for me but it can't be done here"
"Where else can we go Libby?"
"Let's find somewhere, and hope that Alex doesn't get lost"

So we went to find an empty classroom, we didn't find one, but we kept looking, until we found a room, at the time we didn't know that it was the room of requirements, since nobody really went in there.

"Let's go in here"
"I don't know about it Libby, we don't remember how to get out"
"Alex, no offence but you could get lost in a wardrobe"
"What's a wardrobe?"
"It's where you hang your clothes"
"Oh, ok Libby"
"Libby, what was it that we needed to do?"
"I want both of you to promise me something"
"You want us to do an unbreakable bond?"
"No Alex, that's not what I'm asking you to do, I want you to pinky promise that we'll always be best friends, forever and ever"

Alex was scratching his head because he was confused, but then again if the only promise you have ever known is an unbreakable vow... You would be confused too.

"How do you make a pinky promise?"
"You hold out your pinky finger, then you link it with another person's pinky finger and then you promise something"
"Libby, is it a muggle thing?"
"Yes it Alex, Tali can vouch for that"
"I'm vouching!"

Alex still looked so confused, he didn't say that he was but you could tell.

"Let's do it!"
"Wait, how do you know that the other person won't break that promise?"
"You don't, but the guilt will eat at the other person if they do"
"Ok"