You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
Six 6-sided dice are rolled.  What is the probability that exactly two of the dice show a 1 and exactly two of the dice show a 2?  Express your answer as a common fraction.
user
test