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Moodyz Well, in addition to slimes or some chemicals melting heroine's clothes becoming almost a cliche at this point in adult anime, I believe there once has been the online myth of water soluble swimsuit really existing for pranking people. Just like more famous myth of x-ray goggles that can see through clothes if you've heard about it.
Search the video titled "melting swimsuit prank" on Youtube. (CAUTION: NSFW!) I doubt the myth is real and the video obviously looks fake, but I guess some people in Japan want to fetishize on such delusion even AVs look fake like the one on Youtube.