These guys ran an advertisement for glowing sand and pebbles with free samples, so I ordered the sand. Charge it in sunlight for 8 minutes and its good for 12 hours. They show various uses like on paths and swimming pools. Seems pretty neat.
I was tossing over ideas and my buddy came up with the same one I did: put the larger rocks along the fence of your yard and you can tell from the occlusion if someone is inside your perimeter.
I knew a guy who worked in military radar development and he figured out that you could track the B2 bomber by its “shadow” of FM radio reception and conveniently the network is already built out. He did some simulations and proved out the viability. Same idea!