A clepsydra is a water clock. The oldest reliable timekeepers on earth, they told the hours by the regulated flow of water from one vessel to another — from Karnak in Egypt to the great astronomical clocks of Han and Song China. This is a multi-level clepsydra: two real, physically simulated bodies of water linked by a falling stream.
The two cups are offset: the upper cup sits up and to the left so its right-hand lip overhangs the mouth of the lower cup. Over each minute the upper cup tips a little further — gently at first, then steeper — until, near the end of the minute, water runs from its lip and arcs down into the cup below. On the minute boundary the upper cup snaps upright and fills again for the next pour. Meanwhile the level in the lower cup is the dial: it reads the minutes past the hour against the etched graduations, with a float riding the surface as a pointer, and it empties at the top of each hour. The digital readout mirrors your exact local time.
Each body of water is the true geometric intersection of its vessel and a world-horizontal surface, so both surfaces stay level — and remain level even as the upper cup tilts, with the water pooling toward the low side. The pour is launched from the actual rotated lip and arcs under gravity into the lower cup, breaking into droplets and splashing where it lands. The cups are drawn offset, rather than stacked, precisely so the water can pour from a lip into the vessel below the way it would in the real world — a centred drip would not read as a true pour.
Drag the upper cup left or right to tilt and pour it yourself; let go and it returns to the time. Hold Pour to speed the flow, Reset to return both cups to the time. On a phone, tap Motion and tilt the device. Tap anywhere on the glass to cycle the water's tint.