You must guess the computer's code, made from the symbols at the bottom of the screen.
Choose symbols from the bottom and place them on the line the arrow points to. When you are happy with your guess, click "OK".
The computer will show how many you got right (white) and how many were not in the correct position, but are present in the code (purple).
Crack the code before you run out of guesses! Complete the code quickly to get a hi score!