Mila Kunis says ''communication'' is the secret to her successful relationship with Ashton Kutcher.

The 'Bad Moms' star - who has three-year-old daughter Wyatt Isabelle and 11-month-old son Dimitri Portwood with the 39-year-old actor - believes talking openly with her spouse is so important to strengthen their relationship.

She told E! News: ''Communication is key.''

It comes after Ashton confessed he and Mila had an ''agreement'' to keep their relationship purely sexual when they first got together.

He shared: ''She's like, 'You're not ready to be in a relationship' and I'm like, 'I'm not ready to be a in a relationship.' It was funny because I'd just done this movie called 'No Strings Attached' and she just did a movie called 'Friends With Benefits' and we legitimately lived out our movies, which was virtually the same movie. At the time, we were like, 'OK, we have an agreement.'''

Ashton revealed they started to get intimate when Mila, 34, wanted to breathe in the smoke from his cigarette and she had just quit smoking.

He added: ''I was smoking cigarettes at the time and I was still a smoker and she had quit smoking and she wanted me to like shotgun cigarette smoke so she could breathe it in. And I was like, 'Well alright.' And so I started doing this and gradually through the night, it got closer and closer and closer. I think it was pretty mutual - it was kind of obvious that things were happening.''

Ashton and Mila - who got together back in 2012 before tying the knot in 2015 - are determined to keep their children out of the spotlight for as long as they can.

Ashton shared: ''We have a private social network that we share stuff with the families so the grandparents can see the kids and stuff.

''But we don't share any photos of our kids publicly because we feel that being public is a personal choice.''