Numbers typed on a calculator that spell words when read upside down.
On an old seven-segment calculator, certain digits look like letters when you turn the
display upside down. Type a number, flip the calculator, and it can spell a word. The set
of letters that work spells the name of this trick: B, E, G, H, I, L, O, S. Enter a word to
get the number that spells it, or enter a number to read the hidden word.
See more at
Wikipedia.