Actor Ben Affleck.

Ben Affleck has taught his son about "his bits".

The 41-year-old actor - who has daughters Violet, eight, Seraphina, five, and two-year-old son Samuel with his wife Jennifer Garner - was "psyched" when he welcomed his little boy into the world in 2012, and he has even been teaching the toddler about his male private parts.

Jennifer explained: "He loves his girls, but he's serious about his son right now. So, he taught him about his bits. "He taught him about his P and his two Ns.

"Now anytime we pass a dog who's walking on the street or we're at a zoo or something, our poor little son is crouching down and looking up."

The 'Dallas Buyers Club' star, who married Ben in 2005, admits her beau is the "fun" parent and she's stuck doing the mundane tasks -such as cooking - for her children.

She explained to Jimmy Fallon on the 'Tonight Show' last night: "We are divided down a very mum-dad [line]. If it has to do with making food, or homework or anything that has to do with actually functioning, that's a mum thing.

"Then coming in, just as you're getting them to bed and picking them up by their heels and swinging them around? That's a dad thing. Anything fun, that is Ben!"