If your chicks are eating a commercial chick starter feed, they will not need grit. (I'm not sure if this is because chick starter is designed such that it doesn't need to be ground up in their gizzards, or because it contains the correct amount of grit in the preparation.)
Instead of teeth to grind their food, chickens have a special organ called a gizzard. When a chicken pecks up a bit of food, it goes straight to the gizzard before heading to the chicken's stomach. The gizzard is filled with bits of inorganic material like grit, sand, and (in adult chickens) small pebbles. These grind up the food, in the same way that a pepper mill grinds up peppercorns.
At some point, you will want to start feeding your baby chicks some greens, or possibly bugs that you catch in the yard. This is an excellent feeding supplement for your chicks, and it helps introduce them slowly to the Big Wide World in which they will one day find themselves. And frankly, it's really fun.
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