What makes a friendship last?

Sustaining friendships -- and making new friends -- can be challenging, even when we know it's important. So how can we make our friendships last without feeling overwhelmed? And is there a formula to follow for making new ones? To find out, data journalist Mona Chalabi turns to her most trusted sources: data and her mum. Want to hear more from Mona? Check out her podcast Am I Normal? with Mona Chalabi, from the TED Audio Collective.