Onsen
Bathers at an onsen are there to relax, not to get clean.  To keep the water from getting dirty, people at an onsen must shower before getting into the water. In very cold weather, it is sometimes all right to get into the onsen just for a short while to warm up, then go shower, then come back. Some onsen allow bathing suits, but most of the time, visitors must wear no clothes at all.  This is to keep any clothes that might be dirty out of the water, and keep the water clean.  When walking to the onsen, people cover their private parts with a small towel and then put the towel on their heads after they get into the water.  People are not supposed to put their heads or hair in the water, and they are not supposed to have tattoos.  This is because, in Japan, people think of the Yakuza criminals when they see tattoos.