Did you solve it? Maths on the back of an envelope

The solutions to today's puzzles and the results of the pint mnemonic challenge

Earlier today I set you four puzzles, and a challenge to devise a rhyme about a pint being 568ml. The solutions first and the best pint-sized poems afterwards.

1) A piece of paper is folded to make a shape that looks a bit like the back of an envelope, illustrated below. If the paper is unfolded again to make a flat sheet, what shape will it be? Prove it.