This is a fun trick that you can use to trick people into thinking you can read minds. Of course, the savvy observer may be able to deconstruct this trick without much effort, but it still serves as a fun joke. Or maybe it doesn't. It really helps to be twelve years old. On to the trick!
Tell your gullible friend that you want to read his mind. Ask him to choose a number between 1 and 10, and to keep this number secret.
Direct your subject to multiply this number by 9. Then instruct your subject to add together the 2 digits of the resulting number. For example, if you start with 3, 3 times 9 is 27. Two plus 7 equals 9.
Tell your subject to subtract 5 from this number. Then ask your friend to think of the letter of the alphabet with the same rank as his number. For instance, A is 1 and Z is 26.
Instruct your friend to think of a European country with a name beginning with that letter. Once the country has been chosen, ask him to think of an animal whose species' name begins with the last letter of the chosen country. Finally, ask him to choose a color with a name beginning with the last letter of the animal's name.
If your brilliant plan has succeeded, your subject will be thinking of an orange kangaroo in Denmark. If your friend is awed and amazed with this revelation, congratulations.
Here comes the science: No matter what number between 1 and 10 is multiplied by 9, the sum of the 2 individual digits of the resulting number will be 9. So by subtracting 5, this number will always be 4. This means the country letter will always be D. Denmark is the only European country whose name starts with D. The kangaroo is usually the most obvious K-word animal, and orange is usually the most obvious O-word color.
If your mark picks a koala bear or some other K-word animal, you're busted. If your mark pick ochre or some other O-word color, you're also busted. It would be wrong to assume that everyone knows that Denmark is a country, or is in Europe, or even exists. Save your fart jokes for these people. This trick has been around for a long time, and it only works once. So don't get your hopes up.