The way you spend time with friends may morph dramatically over the years, but one thing stays the same: The types of friends who will enrich your life the most.

“There are three kinds of friends that everyone needs in their life: Someone to talk to, someone to depend on and someone to have fun with,” says William Rawlins, the Stocker Professor of Interpersonal Communication at Ohio University and author of The Compass of Friendship. “In my profession, I talk to a lot of people, quite deeply, about their friendships. What struck me is that, no matter how old someone is, our expectations for our friends remain the same.”

Rawlins lays out the key traits associated with each type.

Someone to talk to

Whether you’re having second thoughts about your career path or are worrying about whether that guy is going to text you back, hashing things out with a best friend can help guide you through the problem.

“Acceptance is a cornerstone of friendship—you can open up and be vulnerable with friends,” says Rawlins. “At the same time, when you ask for her opinion, she offers compassionate objectivity: She’ll push you to be the best you can be.”

This person is more than a sounding board. Her role is to help you become a better person, which may mean giving you tough love when necessary to guide you in the right direction. You should be able to let your guard down and confide in her but know she’ll still challenge you in order to help you grow.

Someone to rely on

A big part of friendship is knowing you have someone you can count on for the everyday stuff: Picking you up from the airport when your ride suddenly falls through, lending you a pair of shoes for an interview or watching your kids for an hour during a doctor’s appointment. Knowing she’s always got your back brings a lot of comfort and gives you a sense of stability.

It works the other way, too: Just think about how satisfying it feels to be able to lend a hand to someone you care about.

“Asking a friend for a favor provides her an opportunity to give to you,” says Rawlins. “At the heart of friendship is a desire for the other to live well. If you can help a friend be happy, that in turn makes us happy.”

Someone to have fun with

It’s a no-brainer that we like friends with whom we can have a good time. But there’s a singular pleasure to letting loose with your closest companions, compared to hanging out with your significant other, coworkers or family.

“Our opportunities for connecting with friends are a measure of how much freedom we have in our lives,” says Rawlins.

When you have a friend who doesn’t come with the same types of obligations you have to your spouse or your colleagues, you’re able to have a blast care-free.