Those scratch-and-sniff cards that energy companies send to customers to teach them to recognize the artificial smell added to natural gas work pretty well.Energy West general manager Nick Bohr in Great Falls says that workers recently discarded several boxes of expired scratch-and-sniff cards. But when the garbage truck picked them up and compressed the load it was the same as if they had scratched them. The resulting odor prompted numerous false alarms and building evacuations as the garbage truck traveled through downtown Great Falls on Wednesday morning.