What causes people to become senile?

Louise, realizing that Lady Haloran is emotionally overwrought and superstitious, devises a plan intended to convince the old woman that Kathleen is trying to communicate with her from beyond the grave. This plan involves stealing some of the dead girl's old toys and placing them at the bottom of the estate's pond where they will float to the surface in the middle of the day in an ostensibly ghostly way. At night, Louise swims into the pond and begins placing the toys as planned. However, she is shocked to see what appears to be Kathleen's perfectly preserved corpse at the bottom of the pond. Horrified, she swims to the surface...and is abruptly axed to death by an unknown assailant.