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![](http://mapawatt.com/sites/default/files/styles/thumbnail/public/waterpebble.jpg?itok=r8_tDlXU)
From the Waterpebble website:
"The clever device monitors water going down the plug hole when you shower. Memorising your first shower and using it as a benchmark, Waterpebble then indicates, via a series of ‘traffic lights’ flashing gently from green through to red, when to finish showering. ...