MAGIC.

I can guess your birthday in 10 steps:

1. Multiply the number of the day on which you were born by 5.
2. Add 17.
3. Double the answer.
4. Subtract 13.
5. Multiply by 5
6. Subtract 8.
7. Double the answer.
8. Add 9
9. Add the number of the month in which you were born.
10.Subtract 203.

11. Be so afraid. :D