Iron Man star Robert Downey Jr has issued a challenge to his 20 million Instagram followers in support of a terminally-ill Scottish boy.
The Hollywood actor posted a video of himself wearing a "See You Jimmy" wig and jumping in a muddy puddle.
Downey Jr then introduces Aaron Hunter, from West Dunbartonshire.
The eight-year-old started the challenge to raise awareness of his rare illness called ROHHAD, for which there is no cure.
The “Muddy Puddle” challenge has been issued... Film yourself jumping in the biggest, muddiest puddle possible for 33 seconds(keep it fun and safe) Then challenge 3 people to do the same... Please post the challenge to your social media platform and tag #GOMADJUMPFORROHHAD... No puddle no problem ... Follow the same rules minus the puddle... Remember have an awesome (safe) time... To donate click the link in my BIO or go to crowdrise.com/rdj Calling out my fellow #OGAVENGERS #ChrisEvans #scarlettjohansson @chrishemsworth @markruffalo @doncheadle @renner4real @samuelljackson @paulbettany @elizabetholsenofficial #anthonymackie and to the rest of the #MCU @prattprattpratt @brielarson @wongrel @chadwickboseman @danaigurira @davebautista #beniciodeltoro #benedictcumberbatch #kevinfeige #bradleycooper #loudesposito @evangelinelillyofficial #martinfreeman @gwynethpaltrow @wellhayley @idriselba @jonfavreau @joshbrolin @karengillanofficial @letitiawright @pom.klementieff @jamesgunn @therussobrothers @thejudgegunn @imsebastianstan @therealstanlee @marvelstudios @twhiddleston @tomholland2013 @vindiesel @winstoncduke @zoesaldana #paulrudd ( 🎥 @jimmy_rich 🙌🏼 @mo_freek ) special thanks to @vanhalen for your continued inspiration... and to @rohhad_association_charity for your love and kindness #randomactfunding
The extremely rare condition causes children to gain weight and develop breathing difficulties.
Aaron used to love getting muddy in puddles but it is no longer safe for him to run - and it hurts when he jumps.
He launched the challenge nearly two years ago and it has now been picked up by one of Hollywood's biggest names.
Downey Jr challenged his Instagram followers to film themselves "jumping in the biggest, muddiest puddle possible for 33 seconds" and then challenge three other people to do the same.
He said they should post the challenge to a social media platform with the tag #GOMADJUMPFORROHHAD...
Downey Jr's video was filmed in London, with Aaron and his family looking on in the background.
The Avengers star is dressed all in white and wearing a red hair wig and tartan hat, often dubbed a See You Jimmy wig, as he splashes around to Jump by Van Halen.
In his post he challenges all his Avengers co-stars - including Samuel L Jackson, Scarlett Johansson and Chris Pratt - to get involved by joining the fight against the disease.
The video has been viewed more than 5 million times on Instagram.
Aaron got to meet the star after he posted a video online pleading to meet Iron Man.
To his surprise, after the #AaronNeedsIronMan campaign went viral, the Avengers star contacted him via FaceTime.
ROHHAD stands for Rapid-Onset Obesity with Hypothalamic Dysfunction, Hypoventilation and Autonomic Dysregulation.
The condition affects approximately 100 children worldwide - meaning children have a one in 70 million chance of developing it.
Aaron needs a ventilator to help him breathe.
He calls it his superhero mask because it makes him feel like his idol, Iron Man.