Robert Downey Jr. posted a video to his Facebook page on Wednesday that's pretty surprising if you don't know what's going on.

The Iron Man actor donned an all-white outfit and stepped into a muddy puddle to the tune of the Van Halen song Jump. He then danced about and issued this challenge: "All Avengers and Iron Man fans alike, I hearby challenge thee, go mad, jump for ROHHAD."

GOMADJUMPFORROHHAD The “Muddy Puddle” challenge has been issued... Film yourself jumping in the biggest, muddiest puddle possible (keep it fun and safe) for 33 seconds...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...and don't forget to donate at crowdrise.com/rdj. Calling out my fellow #OGAVENGERS Chris Evans #scarlettjohansson Chris Hemsworth Mark Ruffalo @doncheadle @Renner4Real Samuel L. Jackson Paul Bettany Elizabeth Olsen Anthony Mackie and to the rest of the #MCU Chris Evans @brielarson @wongrel Chadwick Boseman Danai Gurira Dave Bautista (Batista) #beniciodeltoro #benedictcumberbatch #kevinfeige #bradleycooper #loudesposito Evangeline Lilly #martinfreeman @gwynethpaltrow @wellhayley Idris Elba @Jon Favreau @joshbrolin @karengillanofficial @letitiawright Pom Klementieff @jamesgunn The Russo Brothers Sean Gunn @imsebastianstan Stan Lee Tom Hiddleston @tomholland2013 Vin Diesel Winston Duke Zoe Saldana #paulrudd ( 🎥 Jimmy Rich 🙌🏼 @mo_freek) special thanks to Van Halen for your continued inspiration... and to Rohhad Association for your love and kindness #randomactfunding Posted by Robert Downey Jr on Wednesday, March 28, 2018

This all started when young Avengers fan Aaron Hunter sent out an internet request in early 2017 asking to speak to Iron Man.

Hunter, 8, has a rare life-threatening syndrome called ROHHAD, which stands for "rapid-onset obesity with hypothalamic dysregulation, hypoventilation and autonomic dysregulation."

Please RT and help Aaron reach IronMan #AaronNeedsIronman he has terminal ROHHAD Disease & has special message 4 fav hero @RobertDowneyJr pic.twitter.com/9fppkTpKvB — ROHHAD Association (@ROHHAD_GLOBAL) January 28, 2017

Hunter is from Scotland, but his request was heard around the world with an assist from the ROHHAD Association, an awareness group raising funds for research into the syndrome, as well as the When You Wish Upon a Star charity.

Inside Edition reports Downey originally talked to Hunter over FaceTime, but now they've actually had a chance to meet in person.

Hunter successfully recruited Downey to his muddy-puddle challenge, modeled after the viral ice-bucket challenge geared toward raising funds for ALS research. The ROHHAD challenge is a bit messier, but that didn't stop Downey from frolicking in a sloppy pool while Hunter and his family watched.

Selfie game on point! Aaron and I are devising ways we can #gomadforrohhad...Donate just $10 bux (https://t.co/ywpvrsvUm3) to win a trip to #hollywood for the #avengersinfinitywar world premiere. Proceeds benefit @Rohhad_Global and #brianortegafoundation. See you there! pic.twitter.com/wouSDCwWHG — Robert Downey Jr (@RobertDowneyJr) March 24, 2018

Downey is also encouraging his fans to donate to the ROHHAD Association for the chance to win a trip to the world premiere of "Avengers: Infinity War" in Los Angeles. The superhero movie opens worldwide on April 27.

Hunter's mom, Lisa Hunter, shared a message from her son on Facebook: "I love you so so much IronMan, your (sic) my best friend and I always knew you would want to help us."