It's not the kind of thing typically posted, and I certainly don't want cute animal stuff on here in general, but I think this video is, if not appropriate for HN itself, then at least appropriate for the HN demographic. I can't imagine how anyone interested in intelligence and in the mind could not watch this video and question whether dolphins are sentient. Maybe it's similar to a dog fetching a ball or whatever, but I think things like that require training, and I'd assume dogs get an instinct-based payoff (maybe it's similar to chasing prey) and/or payoff from the compliments of their owners. The dolphins seem to be doing this of their own volition, for their own entertainment, and I can't imagine how this behavior could have been trained.

(I don't own a dog, and don't have much experience with them, so take my comparison with a grain of salt.)