One person is "it" and chooses an object which is visible from the car. Then "it" says something about the object they have chosen such as "I spy something white" but they should only use one adjective to describe it. Then each person in the car gets a chance to guess the object. If no one gets it then "it" adds one more adjective such as "I spy something white and round". This continues until someone guesses correctly. Sometimes with little ones you have to be very specific such as "I spy something white, round on that big sign right there."