I had trouble working out whether I wanted '1' to just be a vertical stick, or if I wanted it to have the serifs that would make it more distinct from an 'i'. Then I realised, I could do both, and I made my first transforming number!
So, as you can see, this is a serif-style 1, but it can also close up into its shell, and become a sans-serif 1! The colouring definitely helped here, as it let me differentiate the inner layers from the outer.
because the colours would be important, the sketches for this even had colour!