In this game, we play "... is to ... as X is to Y".
Example
Year is to century as 1 is to 100
Square is to cube as 100 is to 1000
Divisibility is to digits as 1000 is to 3
Anything is to its reflection as 3 is to 8
Belt is to loose as 8 is to 0
(Your X = previous link's Y. Your Y = new number. Whatever replaces the dots can be single words or short phrases)
You have complete freedom of choice with which numbers you use, and you can go into negative numbers if it works. But try not to get fractions or weird stuff involved unless it's just too good to resist :P
Topics don't need to be related to math. (Relation can be somewhat loose.)
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Length is to spacetime as 1 is to 4