Mariska Hargitay & Peter Hermann on the secret to their lasting marriage (Exclusive)

After 13 years of marriage, Mariska Hargitay and her husband, Peter Hermann, are opening up about the key to their long-lasting love -- and how sports analogies have helped!

“There is no secret,” Hermann, 50, told ET during a sit-down interview with Hargitay to promote his new children’s book, If the S in Moose Comes Loose. “We’re all just working through it -- whether it’s a relationship that exists in the public eye to some degree or doesn’t. Our son plays basketball and his coach says, ‘Fundamentals, fundamentals, fundamentals,’ and I think when it comes to relationships, the fundamentals, in the end, are not that complicated.”

“That doesn’t mean that they’re easy, but they’re not that complicated,” he added. “It's fundamentals -- kindness, listen well and fight fair."

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The pair tied the knot in 2004 and juggle parenting their three children with Hargitay’s long-running role as Detective Olivia Benson on NBC’s Law and Order: SVU and Hermann’s latest gig as the love interest of Sutton Foster's character, Liza Miller, on TV Land's Younger.

Hermann also just launched his book, which he started back when their eldest child, 11-year-old son August, was young.

“The first time that I got the galley [proofs], I spread them out all over my table and the first thing [the kids] did was climb all over them, so the pages have their blessing,” Hermann said. “It was funny because the first copy that I got, we sat there and looked at the envelope for a while and very slowly opened it and just had a quiet moment.”

As for how the children responded to seeing their names in ink in the book’s dedication, Hargitay still recalls the look on their faces and said it was a special moment for the family.

“We have these crazy tea parties that are just crazy. It's hard to explain, but the point is for the kids to see the book and also to see their names in the dedication,” Hargitay shared. “[It] was so deeply meaningful to them and I remember their faces … To be included in that was so cool and to know what a huge inspiration the kids were for Peter's fertile imagination was so fun because they know that so many things happened [and] little things they say were included. So that was really cool for them.”

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