1. it had to have a button or lever that set it off once you let go (as opposed to a switch that you just flick)
2. it needed a delay between letting go of the button and the siren going off.
3. the container it was in had to be relatively small and cylindrical (so it could be rolled)
and last of all it needed an led (just to make it look cool).
So this is what i came up with.