I really like this interactive RGB light by Tracy Tsang. Touch the water quickly and the light turns on and off. Stir it up a bit longer and you can select the color of the light. Looks like a lot of fun.

Using a capacitive sensor(QT118H from Farnell), the top of a sweetcorn can was used as an electrode and placed inside water. The water extends the capacitive field of the metal, thus finger completes the circuit when it is in contact with the water.

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