Rice cereal is easy to digest and unlikely to cause an allergic reaction. Alternatives, particularly if your baby has a tendency to be constipated, are barley and oat cereal. (Avoid wheat at first, as wheat allergies are fairly common.)

Instant, ready-to-mix rice cereal is widely available, and the standard brands are basically alike. To prepare instant rice cereal, mix about two teaspoons of the cereal with breastmilk or formula (if you're using a cereal that already includes powdered formula, all you need to add is water). Experiment with the texture to see what your baby prefers, but start out with the mixture a bit soupy (but not so thin it runs off of the spoon).

You can also make rice cereal from scratch, but the downside is that homemade cereals are not iron fortified, and at six months your baby is ready for a boost of iron. The cereal-making process can be as simple (rice plus water cooked into a mush) or as elaborate as you choose. Talk to your doctor about whether your baby needs the extra iron fortification, especially if he's being fed with iron-fortified formula.