Solution to Challenge 1

Folding the practice model
1. Fold a preliminary base (Origami and Paper Folding 3) and hold it with the open end towards you.
2. Take one narrow point. Fold it up at a steep angle, crease sharply, and unfold.
3. Open the point slightly, then push the tip into the inside of the model. The crease flips and the point now faces the other way, with its folded part hidden between the layers. This is the inside reverse fold.
4. On a different point, make the same crease and unfold. This time open the model along its spine and wrap the tip around the outside, so the folded part sits on the outer surface. This is the outside reverse fold.

How the two results look different
The inside reverse fold leaves a clean, narrow point with the extra paper hidden inside. The outside reverse fold leaves a thicker point with the extra paper visible on the outside, and the colour of the other side of the paper shows. If you use two-tone paper, the outside reverse fold shows the second colour on the outside and the inside reverse fold does not.

Checkpoint: both points are symmetric, the creases are sharp, and the model still sits flat.