If we suppose that the shampoo brand my wife asked me is on the shelf, but I can't remember which brand she uses, each of the nine shampoo brands have the same chance of being chosen. So, because there are nine shampoo brands, the probability of getting the right brand is 1 out of 9, i.e., 1/9. In this case we can suppose that the brand was chosen at random because we can't remember the brand she want.
