Other studies tell a similar story. Babies appear to learn language faster when they have parents who treat them as conversation partners. Preschoolers grow larger, more impressive vocabularies when their caregivers sprinkle their everyday talk with a variety of interesting and sophisticated words. The results have held up even when researchers controlled for socioeconomic status and pre-existing cognitive differences between children.

But it's not just a question of being exposed to language. Recent studies suggest that merely overhearing words is not enough. Kids need to be involved in the dialogue. In one experiment, preschoolers picked up new words they heard spoken by an adult, but only in the context of a real, back-and-forth conversation. If kids simply heard the adult talking – if the adult wasn't interacting with them – the lesson didn't stick.

So the case is convincing. Two-way communication matters. But what does a good baby conversation look like? Here are some evidence-based tips.

1. Don't worry if your baby talk sounds a little…babyish.

If you're like many people, your voice changes almost automatically when you talk to babies. Your voice rises in pitch and becomes more musical. You speak more slowly, exaggerate your pronunciation, and repeat key words. The results might make you sound like Mickey Mouse, but they're likely to help your child learn. Studies suggest that babies pay more attention when we speak this way, and have an easier time picking out the new sounds and words they hear.

2. Take advantage of your child's curiosity.

Babies are more likely to learn the name of an object when they are playing with it or otherwise showing curiosity. When your baby looks at something new or interesting, that's your cue to start talking about it.

3. Be expressive – with your body and well as your voice.

If you've ever tried communicating with somebody who speaks a different language, you know how helpful gestures can be. By "talking with our hands" we can help our listeners understand the meaning of new words, and it's a trick that works on children as well as adults. Parents who are particularly good at nonverbal cues appear to help their children learn new vocabulary at a faster rate.

4. Expose toddlers to a rich diversity of words.

Some people tend to "dumb down" their vocabulary when they talk to little kids, but research suggests this robs children of opportunities to learn. Is your child old enough to be interested in the fact that owls don't sleep at night? Then he's probably ready to learn the word "nocturnal." By 30 months, most kids will profit from hearing such specialized words.

5. Talk to preschoolers about the past, future, and make-believe.

When we speak to young children, we often focus on the here-and-now. Do you want more water? Let's put on your socks. But research shows that children – especially older toddlers and preschoolers – benefit when we talk with them about the past and future. So as your child gets older, engage her in conversations that go beyond the immediate practicalities. Discussions about other times and places – including make-believe scenarios – can enrich her vocabulary and boost her language skills.