making a rain detector that will set off an alarm when
it starts to rain.
See the flash animation
a) a plastic bottle or tube,
b) sticky tape,
d) sugar cubes; and
e) some marbles.
Rest a tray or a board on a brick to make a slope.
Glue a row of sugar lumps near the top of the board. ( Or you could
try small heaps of salt if you want the experiment to work fast.)
Put some marbles in the container, seal the end with sticky tape
and rest it on the sugar lumps.
When it rains, the sugar cubes will dissolve in the water and the
container will roll noisily down the slope.