Solution to Challenge 2

Steps (checkpoints in brackets)
1. Pre-crease both diagonals, then both centre lines, and open flat (X and plus sign across the square).
2. Fold each edge in to the centre crease and unfold, one at a time (4 by 4 grid of creases).
3. Raise the walls, fold the corner squares in half along their diagonals, tuck the corners flat against the walls (a tidy open box).

Measuring
- Base should be half the side of your square, and wall height a quarter of it.
- For a 150 mm square: base 75.0 mm, wall 37.5 mm. For a 210 mm square: base 105.0 mm, wall 52.5 mm. A millimetre or two of difference is normal, because the thickness of the paper takes a little off the inside.

Script changes
for s in (100, 200):
    print('Square', s, 'mm -> creases at +/-', s / 4, 'mm from centre; base', s / 2, 'x', s / 2, 'mm; walls', s / 4, 'mm high')

This prints:
Square 100 mm -> creases at +/- 25.0 mm from centre; base 50.0 x 50.0 mm; walls 25.0 mm high
Square 200 mm -> creases at +/- 50.0 mm from centre; base 100.0 x 100.0 mm; walls 50.0 mm high

Answer: when the paper is twice as wide, the base is twice as wide and the walls are twice as high. Every length of the box doubles, so the volume is eight times as big (2 x 2 x 2).