It's National Science Week (although the Cambridge Science Festival is offset by a few days).
At school, the challenge was to build something that could tell you when 30 seconds had passed. The playground this morning was full of a varied assortment of devices, often with two bottles forming some sort of egg timer, with a few marble runs to make it interesting.
Amanda had a slightly more sophisticated variation. Water pours into a cup, causing a float to rise. The float is covered in silver foil and when it rises far enough it touches a metal contact to complete a circuit and turns a light on.