I love this one. :3
And I'm pretty sure everybody's heard of it, but I shall explain anyway:
The first person gives a name. The second person must give a name that starts with the same letter as the last letter in the name that was posted before them. This continues forever.
Exampble:
Poster A: Carl
Poster B: Lisa
Poster C: Alice
Poster D: Ethan
I'll start:
Courtney

