Dame Helen was discussing accents with the star when he cheekily wondered aloud about what she’d sound like after inhaling helium.
Helen Mirren And Jimmy Fallon
Lo and behold, Jimmy then produced two balloons from underneath the desk, and the pair proceeded to continue the interview while inhaling the gas.
Helen even managed to give a full, hilarious award acceptance speech, before the conversation took another cheekier turn…
Of course, US talk shows provide us with plenty of funny moments and James Corden is currently delighting viewers across the pond with his show.
The presenter has secured a number of high-profile guests and so far we’ve seen him posing in his pants with David Beckham, singing with Mariah Carey and reenacting all of Tom Hanks’ films with the man himself - in just eight minutes.
(Obviously, you shouldn't try inhaling helium at home.)
Watch the full clip below...