The large grains inside this salt shaker are easy to mistake for something added accidentally. They are uncooked rice, part of a familiar kitchen habit intended to help salt pour more freely.
The traditional idea is that rice may help with dampness and clumping. Because the grains are larger than the shaker's holes, they stay inside while the salt passes through. It is a household practice, not a guarantee that salt will never cake.
Moisture is worth paying attention to regardless of whether you use that trick. Morton advises storing salt in a cool, dry place away from cooking activity, and notes that salt without an anti-caking ingredient may form lumps when exposed to humidity. Keeping the shaker dry remains the more basic habit behind this small kitchen mystery.
Sources: Morton Salt: Expiration and Storage Guide Original linked story