So a while back there was a thread called "How well do you know your MC?" which was great fun! Basically, every poster on this thread responds to a question posed by the previous poster and writes a question for the next person. The questions then were about your main character, these are gonna be world building. You can respond either with the bare bones answer or use a question as a prompt to write a little short story.
For example, we have the question "How are criminals dealt with in your world?" so you could answer by saying "Primarily fines and incarceration, but some regions have corporal punishment" or you could describe the backstory of your world's equivalent of Alcatraz, or you could write the last day of a man about to be executed via battle to the death with giant scorpions.
So to start us off: What is the most valuable substance in your world?