Number Game - Excellent Number Trick

Did anyone visit http://www.learnenglish.org.uk/games/magic-gopher-central.swf. If you like simple but excellent tricks visit and enjoy.

If you did play the game visiting the british counsil website, some of you may have picked it up what is happening, but if you didn't then read on.

In this game what they are using is a simple number trick.
It is like this,

You first have to think of a two digit number.
For example we'll take 10,

If you add 1 and 0 then it'll be 1.
Now reduce it from 10 so the result will be 9.

To explain the process i'll take more numbers,
10 -> 1+0=1 -> 10-1 =09
13 -> 1+3=4 -> 13-4 =09
15 -> 1+5=6 -> 15-6 =09
19 -> 1+9=10 -> 19-10 =09

21 -> 2+1=3 -> 21-3 =18
25 -> 2+5=7 -> 25-7 =18
27 -> 2+7=9 -> 27-9 =18

33 -> 3+3=6 -> 33-6 =27
36 -> 3+6=9 -> 36-9 =27
38 -> 3+8=11 -> 38-11 =27

Did any one note anything?
There is always a common number coming for each 10 numbers from 0-9.
00-09 the common number is 0.
10-19 the common number is 9.
20-29 the common number is 18.
30-39 the common number is 27.
40-49 the common number is 36.
50-59 the common number is 45.
60-69 the common number is 54.
70-79 the common number is 63.
80-89 the common number is 72.
90-99 the common number is 81.

In the game what they does is they will put an image for all these numbers in there symbol board. So what ever number we guessed we will end up with one of the above numbers and all the above numbers will have the same image on them. Then the program displays the common image as our symbol saying that it read our mind.
The game ends like that but anyway I think it is better if people had the ability of reading other peoples' minds.