This is actually the sandwich problem, which states there is exactly one slice that will split a sandwich of 3 elements into exactly 2 halves regardless of the shape or position of those elements. We don’t need the full proof, but the problem is continuous, so any desired ratio is possible, therefore you will always be able to slice an apple into exactly 1/3 and 2/3rds “good bits”, so a single slice will always be able to do the job.

And that’s why

If you were wondering, it got named “double u” when u and v were the same letter, V was used at the beginning of words and u ain the middle/end. It wasn’t till much later they were seperated into 2 glyphs for different sounds.