Plants are made of tough materials but animals have become adapted to feeding on them. Insects like locusts have strong jaws for biting into the leaves and stalks of grass. Snails and slugs have tongues with teeth on them which rub like sandpaper on plant surfaces and break them up. Finches have short stout beaks which they use to feed on seeds, and all herbivorous birds have a well developed gizzard (a muscular region between the stomach and intestine).

When birds feed they also take in some grit. When this gets to the gizzard it is used to grind the food as the muscles churn it up. The ground-up food passes on to the intestine where digestion is completed. Herbivorous mammals have teeth adapted for feeding on plants. Mice and squirrels have their front incisors adapted for gnawing on nuts. The teeth keep growing all the time to replace the parts which are worn down as the animal feeds. Larger mammals, like sheep and cattle, have molars and premolars which have ridges on them. When the animal chews its food its jaws move from side to side and the ridges on the teeth grind up the food.