If $y^2= 36$, what is the greatest possible value of $y^3$?
If $y^2=36$, then $y$ is 6 or $-6$.  When $y=6$, we have $y^3 = 6^3 = 216$.  When $y=-6$, we have $y^3 = (-6)^3 = -216$.    The greatest possible value of $y^3$ is $\boxed{216}$.