Jessica Biel Reveals 9-Year-Old Son’s Reaction After Reading Her Book About Periods: ‘He Was So Cool About It’ (Exclusive)

As a boy mom, Jessica Biel feels it's her duty to teach her sons about menstruation.

Speaking to PEOPLE about her debut children's book, A Kids Book About Periods, which seeks to de-stigmatize the topic of menstruation for young readers, the actress and producer opened up about how she's teaching that lesson in her own home.

"My son and I read half of the book for the first time this morning, it was really cool to see him listening," Biel, 42, tells PEOPLE exclusively of her 9-year-old son Silas (Biel also shares 3-year-old son Phineas with Justin Timberlake). "He would read a page and I would say, 'Okay, so this is what's happening.' And I said, 'Okay, this is the period. This happens to me every month. And it'll happen to people in school.' "

"This was... The best part, is he kind of was like, 'Oh, that's weird.' I said, 'Yeah, it is kind of weird, but trust me, it's weird for me too. When it happens to me, it's still weird.'"

Concluding, "And I said, 'Think about it. There's somebody at school that might have their period and they might need your help or support someday, and it's really going to be important for you to understand what's happening so you can help a friend if they need something.'"

Ultimately, Biel wants her sons to understand that when it comes to menstruation, there's only "one option."

"The option is to be compassionate and not be freaked out and support," she says. "I got a piece of it in, but I could see the wheels in his brain kind of turning over, and he was so cool about it."

She continues, "I was like, 'Oh my God, score. We did it.' That was the first step in much bigger conversations and deeper conversations that I hope to have with him. And that's exactly why I wrote this book."

As for other parents with sons, Biel hopes that with her book, parents can be less "fearful of telling them the truth" — and trust that they'll be "cool" and "kind" about it.

"I just hope that this book could be one piece of your toolkit when you're having to discuss these types of things. So no fear, go for it," she says. " I just want all the information so when I leave this world, it's better for my sons, it's better for their friends, it's better for their kids."

Biel announced the book, which was created in partnership with PERIOD., in April and told PEOPLE that it aims to normalize discussion around periods and is an organic way to introduce her kids to it early on.

A Kids Book About Periods is now available.