# Marking the safe cup [duplicate]

My friend just told me this riddle and I cannot for the life of me figure out the answer. But he assures me there is one.

There's a room with 64 cups, one of them is water and the rest are poisoned. In front of each cup is a light when you go into the room, you will know which cup is safe, and you will be able to turn one light on or off to let your friend know which cup to drink. However the lights will be randomly on and off, and you won't know the configuration until you're in the room.

You know the organization of the cups (for example we can just say it's a straight line of them), but you aren't allowed to move the cups, or mark them in any way. And my friend assures me it's not 'leave the light on until the cup is warmer'. Basically, it's a light pattern, not some sneaky unexpected answer.

Obviously you need to establish a code with your friend, but how do you establish a way to tell him which cup in a line of 64 while only being able to change one light?

Sorry if this is the wrong place to post this, if so please let me know where I should post it!