Why makes our skin "prune"

Our skin "prunes" because it absorbs water, causing it to expand. As you may know, when a fabric expands in a fixed area, it tends to wrinkle. You may notice that the skin on your fingertips swells when you take a bath--the wrinkling and "pruning" is a consequence of that expansion of the skin.

Hope this helps!

I think it is osmosis. The transfer of moisture, but I am not sure how it works completley though.