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
Let $S$ be the set of all points $(x,y,z)$ such that $x^2 + y^2 + z^2 \le 25$ and $z \ge 0.$  Compute the side length of the largest cube contained in $S.$
user
test