The most important thing to know is why your child is underweight. While some kids simply don't eat enough, there are many possible reasons, such as infections, food allergies, and intestinal, endocrine, heart, lung, and liver problems. Your child should have a thorough checkup and may need a referral for testing with a specialist.

If your doctor has done that and everything's fine, or if he's told you to up your child's food intake until he makes a diagnosis, tricks to help your child gain weight:

Let him eat fat. Put butter on his bread, grate extra cheese over his pasta, put mayo in his sandwich. If he's 4 or older, try peanut butter.

Limit juice. When kids fill up on juice, they lose their appetite. Too much juice can also cause diarrhea, reducing weight gain.

Make each bite count. Give your child high-calorie snacks like cheese, lunch meat, peanut butter crackers, or full-fat yogurt.

Shake it up. There are high-calorie milk shakes available at grocery stores, but you can make your own by mixing instant breakfast mixes, powdered milk, or yogurt into whole milk. Add fruit for sweetness. Don't worry  -- keeping in close touch with your doctor and playing with your child's diet will help him get on track.