*Common resource: rival, not excludable. Ex. fishing in lake Michigan. *Without government intervention people will use this resource too much, because they don't think of the cost they are creating for others, by finishing the fish and ruining the environment.
Work Step by Step
*RIVAL: If some one is fishing and catches fish, as a result they will leave less fish for the next person. NOT EXCLUDABLE: ocean is free, cannot charge to fish.