One person starts by choosing an object (perhaps a cow) and says "I spy with my little eye, something beginning with the letter C". The other players look around and suggest things it might be: "Crow" (no), "Car" (no), "Cloud" (no), "Cow" - yes. The person who guesses correctly gets one point and also gets to choose the next object.
When played in a car, a family need to decide beforehand what to do about items within the car (usually excluded) and things that are no longer visible (the field with the cows is a mile back - usually permitted but the 'I spy'er has to say "I can't see it any more" and "I can see it now").
A slightly more challenging version uses phrases - "I spy something starting with B A W C" for "Black and white cow".
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