Whenever possible, bring the bike inside with you. If that’s not possible, consider doing your shopping or whatever some other time, because the risk of bike theft is great almost everywhere. If you absolutely have to leave the bike while you go inside for a few seconds, do this: Unscrew the adjustment barrel on the front brake until the brake is locking the front wheel tightly. Now, if some jerk tries to rip off your bike, he at least won’t be able to ride it (because it won’t roll at all). With any luck, he’ll think it’s a bad bike and he’ll leave it alone.