Okay, lets imagine that you go to a misterious house and you find 9 rooms there... We can name it to room A, room B, until room I... Here is how it look:
A B C
D E F
G H I
The rules is: The first room when you get in is A and you can move horizontal and vertical, you are not allowed to move diagonal.
For example, if i told you to move 3 steps, so you can move A-B-C-F, A-B-E-H, A-B-A-B, etc.. Understand? If yes, lets play! Start from room A.
Now move 4 steps.
A B C
D E F
G H I
Then I took B room, because I know you are not here.
Move 5 steps.
A - C
D E F
G H I
I know you are not in room C, so I take it.
Move 4 steps.
A - -
D E F
G H I
You are not in room A and I, I take it.
- - -
D E F
G H -
Now move 7 steps.
I can see that you are not in room F and H. Now I take F and H.
The last, move 5 steps!
- - -
D E -
G - -
AHA! I KNOW WHERE ARE YOU!
YOU ARE NOT IN ROOM E!
YOU ALSO NOT IN ROOM G!
......
YES..., YOU ARE IN ROOM D...!!!
Notes: Use your finger and touch the screen if its hard to remember your last step...
This trick is the one I solved from David Cooperfield tricks. If you have seen the "train" episode from his shows, you must know this magic...