"He's in the trenches and started his potty training," Spelling, 35, told PEOPLE at the Pull-Ups Potty Dance Party in New York. "People say how do you know when your kids are ready to potty train, and I knew because he would come up to me and go 'poo' and just hold himself."
Spelling and husband Dean McDermott's potty training techniques with Liam seem to be paying off.
"His new thing is that he likes to greet people, and he loves the movie Cars. So, he wants to show you cars on his Pull Ups, so he pulls down his pants, and he's like, 'Cars,' "says the proud mom.
"The other day while shopping at Barney's," she recalls, "a fan came up and recognized me and said hi to Liam, and he dropped his pants and said, 'Cars'!"
According to Spelling, Liam will be a great role model for his younger sister Stella, 8 months.
"She's going to be easier to potty train just because she is obsessed with Liam. She does everything he wants and wants to do everything he does. She already wants to walk, and she'll probably be potty trained by the time she's a year old," Spelling gushes.
Post-Baby WeightLooking particularly svelte, Spelling admits that, in regard to baby weight, "I found it harder to lose it the first time."
She says, "It was a daunting task to lose 40 lbs., and I was like, 'How do I do that?' I was a little stressed, and I didn't want to go to the gym."
Her method? "I didn't do any specific hardcore workouts, and I didn't have a strict diet regime. Dean jumped in with the kids, and we all did it together. For physical activity we went biking, jogging, hiking and did everything together.
"If you don't stress about it, you will have fun doing it," she advises. "But towards the end I started doing yoga, but that literally came because Liam would do it. One day I would say, 'Let's do exercise,' and he got into the downward dog pose, and I'm like, 'That's genius' – my baby knows yoga and he's never been taught. So, then we started to do yoga in our back yard."
Back to Old Zip CodeBesides potty training, Spelling returns to her former role (of 10 years) as Donna Martin on the CW's 90210 spin-off.
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Spelling reveals that Donna is now a new mom. "She brings her baby and visits Kelly and Silver from Japan because she's a big fashion designer."
Of reuniting with cast member Jennie Garth, Spelling said, "Seeing Jennie again and all of a sudden we were two giggly school girls."
Spelling – daughter of the original show's producer, the late Aaron Spelling – says her father "would be really proud. He always intended to make a 90210 for the new generation, so I think he's probably going, 'It's about time!' "