This lesson develops the angle-fold pop-up further. Students predict what will happen when they add a second triangle to the Angle-fold Template. They then explore the directions of motion of the two angle-folds. Next they reverse the triangles and again predict what will happen, before trying it.

**For each student**: *Scissors, tape, rulers, ballpoint pen, and Post-itâ„¢**link*

- Angle-fold Template
- Worksheet: Monster & Mouth
- Science notebook
- Folder for saving pop-ups

**The Monster**. Show students how to tape two triangles into the template to make a Monster. Ask students to predict what the Monster will do before they try it. They can use the Worksheet, Part 1.

**Outcome**: When the book opens, the two angle-folds will push their flags in opposite directions, above and below the top and bottom of the book.**How long should the flag be?***Ask students to think about what happens if they make the flag longer or shorter*. After they have made some predictions, they explore how long they can make the flag.

**Outcome**: The longer the flag, the more it will move. If the flag is too long, it will stick out of the book when the book is closed.**The Inside-out Monster.**Ask students to predict what will happen if they turn each of the triangles around, so they are pointing towards each other. They can use the Worksheet, Part 2.

**Outcome**: The flags now push towards each other as the book is opened. As with the Monster, the motion is always on the flat side, away from the vertex, which is the pointed tip of the triangle. The two pop-ups look like the two jaws of a mouth.**Extensions.***Click below to see a video about each of these challenges: Make**curved flags for your Monster*- Make a mouth with teeth
- Make a mouth with a tongue
- Make a dog’s mouth with a bone inside
- Make scary-looking crooked jaws

- Make a mouth with teeth

Click below for suggestions in case …

- the pop-up piece stays in the book and won’t pop up
- the pop-up comes apart when you open or close the book
- the book won’t respect the existing fold or page positions and makes new hinges