For example, lets i throw a soda can at you, to you, you were hit by a ufo, you turn around and find a soda can on the ground, now you know you were hit by a soda can, so lets say now you throw a grenade back as revenge, i get hit by it, and to me, i got hit by a ufo, i turn around and see a grenade, i generously throw it back, but without the pin, you get hit by it, but before you turn around, it explodes and kills you, meaning now it cant be identified since its gone, leaving it as a ufo to anyone except me.