Dave Hallock, age 14, of Eugene, Oregon, for his question:

Why do chickens eat gravel?

Your digestive system needs a certain amount of roughage material in the form of stringy vegetation. The digestive system of a chicken is very different from yours, mainly because the bird has no teeth to chew its food. In both cases the food must be pulverized into soupy mush before its nutrients can be used by the living cells. In both cases, the digestive tracts supply jucies to break up and partially change the nourishing chemicals. The chicken's digestive system must work harder to mash up her digestible mush.

She is a scratching and pecking bird, foraging for her food on the ground. And like all birds, she swallows each morsel whole. It may be stored for a while in her clop. Later it is grabbed by a strange muscle called the gizzard. This tough fist does the chewing job, though it has no teeth. Instead, it uses a few sharp pebbles. The chicken supplies these gritty grinders by swallowing stones. The gizzard holds them in its muscular fist as it squeezes and kneads the food around and around.