Tub Ring Easy Clean

I was taking a bath the other day and noticed that once again there was a ring around the tub. When I got out of the tub I went to grab my cleaner and found that I had run out, but knew that I would forget to clean it later if I didn't do it now. I grabbed my cleaning rag and put a small amount of my hair shampoo onto it and started wiping. The ring came right off with no effort and my tub was sparkling clean in no time.