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Chalk?
Sweaty hands make it harder to hold onto the holds, so chalk is used to keep your hands dry. If you watch some climbing videos on Youtube, you’ll see it all the time.
It doesn’t really matter with beginner holds like the almost cup-like ones Tsugumi used in chapter 1 and you can also do the intermediate ones just fine without chalk, but dry hands are important for the harder holds, like the ones that are more like little bumps that no sane person would actually attempt to hold onto.