To get back for a moment to the "superstitions" list, it occurred to me that it would be interesting to google some of them to see their origins. Here are two that I found:
Where did throwing salt over the shoulder come from?
A superstition says spilling salt will bring you bad fortune. As legend has it, you should take a pinch of the spilled salt and throw it over your left shoulder to cancel the bad luck. The left shoulder was where the devil was supposed to be sitting and tossing salt in the devil's eyes kept the devil away.
(But why does spilling salt bring you bad fortune?)
About knocking on wood, the general belief seems to be that the ancient druids believed that ghosts and spirits lived in trees, and knocking on the trees woke them up so we could ask for their protection.
This could be a whole topic in itself under General Discussion.