How did the DUCK HUNT game work ?

Just a curious question. Remember long time back when we had our first set of consoles with those 64 in 1 types cartridges and games like Super Mario Bros and Contra. In those set there was a game called Duck Hunt where we had a gun which we pointed to the TV screen to kill the flying duck. I am still unclear about how that game worked


PS: It may be a noob question.