Key signatures arrange the "diatonic scale" is a way to fit different tonics.
No Sharps, No Flats = C Major
These intervals are all whole steps EXCEPT for between the 3rd/4th and 7th/8th
If we flatten the B, we change the half steps to between the 3rd/4th and the 6th/7th
If we keep these intervals but change the first note to where the fourth note is...
Add a flat to the 7th step, and the whole process repeats, and now the tonic (or first step) is Eb
Bflat C D * Eflat F G A * Bflat
Add a flat to the seventh step, and the fourth step becomes the tonic!
Eflat F G * Aflat Bflat C D * Eflat
Father Christmas Got Dad And Electric Blanket! - (sharps)
Blanket Exploded And Dad Got Charred Feet! -(flats)