People are made of clay, stone, brass, or hollowed-out wood. (Make sure to choose one and make a persona of it or just repeatedly subtly mention it.) Hearts are made of red yarn.

On everyone's 18th birthday, they're given a soulmate by their hearts.

The person's red yarn heart unravels and one end stays connected to the person while the other connects to the person's soulmate's free loose end.

As you get closer to your soulmate your heart starts to be gathered again back into shape

Most of the time soulmates aren't very far away from each other when they receive one so many people only have to go to the next state over to find their soulmate.

Your 18th birthday was last Tuesday. It's Friday now.

It isn't like you didn't get a soulmate or anything like that... the only thing that is... odd about yours is... Your soulmate string has been going up into the sky ever since your 18th birthday.