The riddle
What Did Zero Say to Eight?
Two digits side by side
Write a 0 and an 8 next to each other. The zero is one closed loop. The eight is two loops stacked, pinched where they meet, and that pinched middle looks like a belt cinched around a waist. To a zero, an 8 appears to be a zero that got dressed up, so "nice belt" is a compliment between two numbers. The joke depends on the shape, which is why it works in print and with a finger drawing in the air.
The belt on a digital display
Calculators and digital clocks draw each digit with up to seven bars. A 0 lights six of them and leaves the middle bar dark. An 8 lights all seven. So on a display, the 8 really is a 0 with one extra bar across the middle, and that bar is the belt.
Try it with a pencil
Draw a 0 as a rectangle made of six short lines, then ask a friend which single line turns it into an 8. The one across the middle does it. Then work backward from the 8 and erase one bar at a time. On most displays, taking away the lower left bar leaves a 9, and taking away the upper right bar leaves a 6.
A number joke that works by ear
Not every number riddle relies on the picture. In why is 6 afraid of 7, the punch line is carried by sound alone, so it can be told in the dark.