alt Hacker NewsSorry to be a bore [that's a firearms joke], but -- while I encourage everyone and their family to experiment with building such devices, with safety in mind of course, it is a lot of fun -- please don't dispose of your old batteries in the natural environment!
Really not trying to call you out here, as a teenager I once had the idea to load up my marble cannon with dead AAs (they're free!) and got a "do you really think that's a good idea?" from an environmentally conscious adult when I went asking for dead batteries.
We moved on to oranges really fast after realizing that batteries coming out of the sky randomly was probably a bad idea, even if there were no apparent people around.
Don't get me wrong, oranges are dangerous too, coming out of the sky at terminal velocity, but at least when they hit the ground they disappeared into a fine, 3 meter wide mist.