@JeremyECrawford if a creature is swimming, hovering or is otherwise unable to fall (e.g. chained to a wall), does knocking them prone actually inflict the prone condition?

— Armando Doval (@armando_doval) February 2, 2018

If it's physically impossible for you to be prone, you're not subject to the prone condition. #DnD https://t.co/5lJTjq4Rli

— Jeremy Crawford (@JeremyECrawford) February 2, 2018