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Giant African land snails will eat almost any vegetarian food, but fresh produce is your best option. Start with foods like lettuce, cucumbers, apples, bananas, and cabbage. Also try corn and bell peppers, as well as zucchini, grapes, melons, watercress, and spinach.  Always check up on the food, and remove it when it's gone bad. Skip onions, pasta (starchy foods), and anything with salt in it. Make sure to scrub your snail's food thoroughly before giving it to your snail. You want to make sure any pesticides are washed off so that your snail doesn't eat them. Place a shallow water dish in the snail's container for the snail to drink. This dish has the added benefit of providing humidity to the tank. Be sure to change it out once a day. Snails need a steady supply of calcium to maintain their shells. An easy way to provide it is to place a cuttlefish bone in the tank, which can be found at most pet stores. Use ground eggshells (clean) or ground oyster shells as an alternative. If your snails don't seem to like these, you can find calcium you can spray on top of their normal food.

Summary:
Choose fresh produce. Wash your snail's food thoroughly. Use a small water dish. Provide calcium to the snail.