A hollow metal sphere is filled with water and hung by a long thread. A small hole is drilled at the bottom through which water slowly flows out. Now the sphere is made to oscillate, the period of oscillation of the pendulum -

First water leak outs from metal sphere due to this COM of the system shift downward therefore effective length of simple pendulum increases. So $\mathrm{T}\propto \mathrm{\ell }$; So T also increases. After that slowly COM again rises upward & due to its $\mathrm{\ell }$ decreases and $\mathrm{T}\propto \mathrm{\ell }$, Then T decreases and finally COM set previous position S. Time period also attain starting value.