Elevator Fountain

This is like a conventional fountain, with a pump that squirts water upwards and a little pond to collect the spewings. Except, it's mounted in an elevator.

A digital circuit controls the pressure of the pump, using a feedback loop, to ensure the height of the fountain remains constant.

As a result, when accelerating upwards, the fountain appears to run in fast-forward, and when accelerating downwards it appears to be in slow motion.