This is a small homemade switch made by sponge and circuit. It is a very simple circuit structure. I just use the sponge, which is conductive when it gets wet and non-conductive when dry, as a part of circuit.
I come up with a lot of ideas.
The first way is to change the conductivity of a material to show the transition of the lighting, and also the transition of the material. For example, melted ice can be a conductor, so do boiled eggs.
The second way is to use some weird and unexpected conductive materials, such as Coca Cola, fruits, and some special behaviors, such as inserting coins to not only make a weird switch, but also a funny video to lighten the led.
Finally, I choose a weird conductive material, sponge and using watering as a behavior to turn the switch on, and then watch the sponge expands and led turns on.