I'd like to believe that Simon Riley is the kind of guy to watch animal documentaries.
One night, he watches one about Gentoo penguins, who mate for life. The males court their chosen partners by presenting them with the smoothest pebble he can find, and should his potential mate accept, that pebble becomes the foundation for the nest they will build together.
While on deployment, Simon searches the ground and rubble for the smoothest, most appealing rock he can find -- it's not like he has much else to do during downtime -- and then he gives it to you.
You don't really know what to make of it at first, but the mere act of accepting it opened the way to receiving pebbles each and every time he comes back from deployment, or anywhere else really. Now you have rocks from all around the world. And also from outside the barber shop and the bakery and the beach....
