Let us start with the youngest. Babies know faces they have seen, and they show signs of being selective about who they spend time with. A baby can also react when another child is upset. At first, very young kids play near each other without interacting much. As they learn language, their play becomes more interactive. Some kids who are not yet in school have one particular friend. Groups of friends can start to form.

Next come the school years. These kids often prefer to play with kids of the same gender. Friendships are more common between kids who like the same things. School-age kids learn to deal with disagreements with friends. Older kids start to form lasting, supportive friendships. By middle school, friends confide in each other, which means they share private things. This is how friendships change across the school years.

Now look at time. Making a friend takes time together. In one study, it took roughly 50 hours together to move from acquaintance to casual friend. An acquaintance is someone you know a little. In one study of grown-ups, a close friend took more than 200 hours of time together. But time together does not automatically make friends. How people spend their time and what they talk about changes how close they become.

Here are two more places to look. One study scanned brains. Friends showed more similar responses to the same movies than people farther apart in their social network did. A social network is the web of people someone knows. Friendship is not only a human thing, either. Female baboons form close bonds with other females in their troop. Baby baboons were more likely to survive when their mothers had close friendships with other female baboons.

Words to know

• acquaintance: someone you know
• confide: share private thoughts
• network: web of people
• troop: animal group