The skin is evidently the largest organ of the body. Amongst other functions, it prevents transepidermal water loss (TEWL). As good as the skin is at preventing dehydration through TEWL, it is even better at keeping things out. Things like water too. This sounds a little strange but imagine you absorb water everytime you take a bath or got rained on, think about the myriad of substances that get on the skin by accident or on purpose, what about bacteria and other grimy things that touch us from time to time. Our skin is highly resilient and resistant to external forces.
With this said, the skin does allow some compounds through based on: