I enjoy discovering secret information—stuff most people don’t know about. And I love spreading the word. For example, my supermarket, like most, offers a rain check if they run out of a product that is on sale. This is really great, in my opinion, because my store’s rain checks have no set expiration date. But there’s another secret: There’s no limit on how many of that item I can get later—at a more convenient time—with my rain check. Case in point: Last summer, this supermarket advertised its London broil cut of beef for $3.99 a pound. That’s a bargain where I live. And sure enough, the store sold out before noon on Sunday, the day I was shopping. No problem. Because my goal was to load up for my freezer, I asked for a rain check. A full two months later, when London broil was at its regular price of …