Why are your hands slippery when dry, get “grippy” when they get a little bit wet, then slippery again if very wet?
In: Physics
This is a question of friction. There are two main types of forces contributing to friction.
First we have literal roughness/unevenness in the surface (termed [Asperity](https://en.wikipedia.org/wiki/Asperity_(materials_science))) which means two surfaces can hook into each other, sort of like velcro, and your hands has a lot of these (e.g. finger prints).
The second is a bit less intuitive and is called the Van der Waals force. This can cause friction between smooth surfaces because the force comes from the atoms and molecules themselves. This is the reason you get a better grip on your cellphone case that is made of soft rubber than you get from the glass and hard plastic of the phone itself.
Beyond this it can often be hard to specifically determine what forces will dominate for a given scenario.^([1]) Climbers generally want to chaulk their hands completely dry to get a better grip on the rough surfaces they climb on. If the surface is really smooth then you might want to rely on the Van der Waals force instead, for instance by using indoor running shoes with soft rubber soles when in a gym with equally soft rubber flooring. However, be careful of any lubricant that can come between the smooth surfaces, as this will remove the Van der Waals force completely and make it extremely slippery.
For your specific example I would conjecture that somewhat wet hands strikes a good balance between using the Van der Waals force and the roughness between surfaces to obtain relatively strong friction for a variety of scenarios. Too wet hands could for example act as lubricant on the Van der Waals force. (Dry hands I am less sure of but I have a feeling it could have something to do with reduced surface contact)
>[1] The relationship between frictional interactions and asperity geometry is complex and poorly understood. It has been reported that an increased roughness may under certain circumstances result in weaker frictional interactions while smoother surfaces may in fact exhibit high levels of friction owing to high levels of true contact.” [wikipedia.org/wiki/Asperity](https://en.wikipedia.org/wiki/Asperity_(materials_science))
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