Have you ever been barbecuing or travelling with you bike or done some other outdoor activity, and you looked at the overcast sky worrying when exactly it was gonna start raining?
Well, when the rain finally hits you, it has already been falling for some time (a minute or possibly more). So I'm wondering if it's possible to create a cheap device that'll detect the exact moment the rain starts falling and warn you. A minute advance notice can be extremely valuable - you can evacuate your CDs, get everyone inside, find shelter, etc.
Reward: One such a device for me (plus free repairs)