The first poster lists a number. The next poster responds by attaching some significance/explanation to that number and then posts a number of their own.
e.g.
A: Seven (7)
B: The number of Wonders in the ancient world. 18
C: The age at which Burpo the Wonder Mutt drank his first (legal) beer. 3.14
D: The number of seconds it take me to eat a slice of my grandma’s apple pie. 112…
I’ll kick things off with the number 12