the Tropism Fountain

The Tropism Well from Poietic Studio on Vimeo.

Saw this video over on Gizmodo and couldn't resist a repost. the Tropism Well designed by Poietic Studio senses someone approaching and very kindly offers them a drink.

I am ridiculously curious how this works, sensors in the ground? cameras? and most importantly how does it know when to stop pouring!?