It sounds simple enough, but shopping for nipples can be confusing. Besides the basic type, there's a large array of options.

"No nipple is inherently better than any other," says Wendy Wright, M.D., a pediatrician at the Mount Sinai Hospital, in Chicago. "The best one for your baby is the one he feels most comfortable with." If an infant likes it, he'll quickly grab on to it and suck vigorously. If he doesn't, he'll let go after a few sucks, his body may tense up, and he may begin to fuss or cry.

What the different nipples you can buy really offer:

ORTHODONTIC The bulb, the part of the nipple the baby sucks on, is flattened to resemble the shape of the mother's nipple during breastfeeding. Because of its shape, this type of nipple sometimes sticks shut after feedings, making it difficult to clean.

ANGLED The nipple leans to one side so that it automatically tilts into your baby's mouth.

VENTED A small hole in the neck of the nipple lets air into the bottle while keeping liquid in. This prevents the nipple from collapsing. While vents help the nipple stay erect, most babies instinctively suck just fine without them by opening their mouth and letting air back into the bottle.

SIZE 1 (or labeled "for babies 0 to 6 months") The hole is smaller, to prevent the baby from taking in too much at once and gagging. The entire nipple may be smaller than those designed for older babies.

SIZE 2 (or labeled "for older babies" or "faster-flowing") A larger hole  -- or sometimes more than one hole  -- allows for a quicker flow.

FOR PREMATURE BABIES This nipple has a tinier hole than others, and is softer, to make sucking easier until a preemie gains more strength.