Prime Number

I think prime numbers are like life. They are very logical but you could never work out the rules, even if you spent all your time thinking about them.
Mark Haddon, The Curious Incident of the Dog in the Night-Time

What to make of this - a small number written on a piece of slate dangling in a tree, surrounded by clooties of seaweed, for the tree is fallen and the branch dangles just above the sea and at every rough high tide is festooned. It has to meant something - doesn't it???
Perfect for Mono Monday, the theme being something small. Thanks to Skeena  for hosting.