All of the Cortex challenge cards have a symbol on the back corresponding to which challenge it is so you always know what the next challenge is going to be. The top card gets turned over and then you check out the challenge on the card. When you think you have the answer you put your hand over the card and give your answer. If you get it right (which you likely will) then you get to keep the card towards a trade in for brain pieces. To make the game a little more of a challenge (and it really needs something) you can only have 4 cards in front of you at one time.