Janet Jackson was brought back to earth during a recent trip to a supermarket with pal Missy Elliott, when she discovered for the first time what shopping carts and credit card machines are. The superstar reveals her astonishing ignorance of the basics of grocery shopping in a magazine interview. Jackson, 40, tells V, "'I haven't set foot in a grocery store in years. And that's so embarrassing. "I kept going 'What's this?' First of all I had the cart and I was riding down the aisles, standing on it. And we got to the checkout and there's this square thing and I'm like, 'What's this, you guys?' "They said, 'That's so you can use your credit card' "And I said, 'You can use your credit card in grocery stores now?' Missy just cracked up laughing.'