This is a simple trick that many people know at this stage. You ask someone to pick a number and ask them to perform a series of arithmetic steps to that number. The trick is set up so that they will always have four as their answer. The next step is to for a European country that begins with the letter that is equivalent to the number you have chosen (a=1 ..).
Now they should be thinking of Denmark and you request an animal that begins with the last letter from that country's name. This will normally be a kangaroo and from there you ask for a colour beginning with the final letter of that word. Noy you can proclaim, "Ah come on! There are no orange kangaroos in Denmark".
Going four->Denmark->kangaroo->orange is simple enough, but how do we make sure that every time out we turn there number into four? You have done it hundreds of times before.