Can someone please explain how gas pricing works?

I don't understand how, within two hours, gas prices can drop 22 cents and then rise 25 cents by the next morning. I don't imagine that gas companies are getting new shipments of oil every hour or every two hours, all of which are different prices. Seriously, why do gas prices change like they do, arbitrarily and often when few, if any, other commodities fluctuate in such a way?