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."
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."
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.
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 .
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."