Constipation -- or hard stool -- is a common problem, especially after a baby starts eating cereals. If you notice that your child's stools are firm and dry, or he's having difficulty passing them, try cutting out rice cereal for a day or two to see if that does the trick.
Another common time for babies to develop constipation is around their first birthday. It's no coincidence that this is also when most parents start children on whole milk. Too much milk can lead to sticky, claylike stools that present a real problem for some toddlers. If you suspect milk is the culprit, try limiting your child's intake to 16 ounces per day.