Why is it bad luck to spill salt?

If you spill some salt while you're cooking, you might fear a wave of oncoming bad luck. Why is toppling over some salt an omen of misfortune?

Why So Salty? - Superstitions about salt

Where do Superstitions come from?

Debra Ronca HowStuffWorks

Why Italians Believe It's Bad Luck To Spill Salt Or Olive Oil

Short Stuff: Why Spilling Salt is Unlucky

How much bad luck will I experience if I tip the salt shaker over

Why Do We Throw Salt Over Our Left Shoulders When We Spill It

Why Is a Broken Mirror Bad Luck?

Why Is Throwing Salt Over Your Shoulder Good Luck?