Scarlett Johansson's son is obsessed with his feet.

Scarlett Johansson has given an update on her baby son

Scarlett Johansson has given an update on her baby son

The 'Black Widow' actress explained four-month-old Cosmo - who she has with husband Colin Jost - has just "realised" he's got toes and joked it's been a "life-changing" discovery as now he can't stop playing with them.

Speaking on 'People (The TV Show!)', she said: "[Cosmo's] great. He's been making all the fun discoveries like feet.

"It's the best. I mean, feet, who knew? There they are. Life-changing.

"[He will] stare at them, pull on them, chew on them.

"It's just unbelievable. I mean, can you imagine you never realized you had feet and then you looked down and there they are? You're like, 'Whoa.' "

And as for her seven-year-old daughter Rose - whose father is the 37-year-old actress' ex-husband Romain Dauriac - the youngster is "pretty excited" to have discovered her mother will be in 'Sing 2', after recently "figuring out" how animated movies work.

Scarlett said: "I think she's pretty excited about the fact that I'm in 'Sing 2'.

"Although it's probably more abstract to her. Like she just recently figured out how animated movies work because that's kind of a surreal, kind of concept."

Bono also appears in 'Sing 2' and Scarlett - who released an album of Tom Waits covers called 'Anywhere I Lay My Head' in 2008 and 'Break Up', a collaborative LP with songwriter Pete Yorn the following year - joked she'd be open to a musical collaboration with the U2 frontman.

She quipped: "I'm only going to do it if Bono does it. Now, he's come to my turf and then if I go to his turf, we got to work on the album together."