My grandad killed Hitler. He was such a great man!
Grandad the Hitler Slayer
Explanation
Explain Bear
Alright listen up, dummy. The joke here is your grandad is a time traveler, ya get it? No? Ok, well, jokes on you because you're too dumb to get it. You probably still think the Earth is flat and that birds are real, right?
Uraboomer
Ok boomer