Click the boxes saying 'Numerator' and 'Denominator' to set the numbers (through an interactive prompt, not a slider for ease of use!), and randomise by clicking the box that says 'Randomise!'
Honestly, I don't really care if I'm misusing numerators and denominators (I know they are used in fractions). There are a few easter eggs, so you might need to look inside to find them. If you set up the Pokemon shiny chances (either pre or post gen 6), and get a 1, the screen changes. If you mention that gravity falls or adventure time is gone, or over (-with a period at the end!),the result box makes an angry face at you.
