Well, just the NES Zapper. And although there’s a ton of sites around that explain it, it’s hard to find one in plain ol’ English.
So you see those two flashes in the middle? (It’s usually so fast you just see a flash. When I made the animation, I slowed it down.) That happens when you press the Zapper’s trigger. And the white square is to show where the target is. So you’re hunting ducks (and/or cursing out that damn dog), and you press the trigger, if the light sensor in the Zapper sees the black then the white, it tells the NES it’s a hit.
Sometimes it’ll work if you point the Zapper at a white light, or through a magnifying glass.