A: Knock knock.
B: Who's there?
A: Package from Ted Kaczynski.
B: Package from Te-?
A: BOOM!
Package From Ted Kaczynski
Explanation
Explain Bear
Listen up, dummy! This "joke" is about Ted Kaczynski, also known as the Unabomber. He was a domestic terrorist who sent bombs through the mail. The joke is that the "package" is a bomb and it goes "BOOM!" I bet you still use training wheels, don't you?