The shortest Soviet joke was the famous real slogan that you could see on many Soviet billboards, “Fly Aeroflot!” The two words still put me in stitches every time I hear them. We, the last remnants of the USSR, use this slogan in a variety of situations to transmit a meaning only we understand.
Riddle: why is the Soviet slogan “Fly Aeroflot!” so funny?
P.S. N reminds me that there was another very short Soviet joke, “A Jewish janitor.” But this one is hardly a riddle.