Creature Lab: How Far Can a Body Bend?

Before you start: try it once yourself

Close the fish's ring yourself once before you start, about ten minutes.

1. Open the lesson. Go to mytekdev.com/try/creaturelab?lesson=how-far-can-a-body-bend&ref=fam-creaturelab-1. A fish, a snake and a lizard swim in tight circles. The selected creature's name is in the box at the top left, next to the numbers 1, 2 and 3; the fish is 1.
2. Look under the skin. Open Skeleton in the rail on the left and pick Bones. The snake's chain coils inside itself, the lizard's just closes, and the fish's stays a C.
3. Make it floppier. With the fish selected, open Body and drag Flexibility to about seven tenths of the way along. The view stays on Bones, and the fish's chain now closes into a ring.
4. Try length instead. Drag Flexibility back to two fifths of the way along, where it started. Then drag Length right a little at a time, counting the dots, until there are 16. The ring closes again.

Bones view with the starter: only the fish cannot close its ring.

The ideas, in plain words

Each creature's body is a chain of joints. The head goes where the brain steers it, and every joint behind it follows one simple rule: stay the same distance behind the joint ahead. Nobody animates the tail; it falls into place because the rule repeats down the whole chain. Game and animation programmers use this same follow-the-leader trick for snakes, tentacles and ropes.

Each joint can also bend only so far from the line of the joint ahead, and the Flexibility slider sets that limit, from about 13 to about 42 degrees. Bends between joints add up, so the whole body can curl at most the number of bends times the limit. A full circle is 360 degrees. A body with too little total bend makes a C however hard it turns. A long body can close a ring even when each joint is stiff, because many small bends add up: that is how a real snake, with more than 200 vertebrae, coils.

Do it together

1. Open mytekdev.com/try/creaturelab?lesson=how-far-can-a-body-bend&ref=fam-creaturelab-1 together. A fish, a snake and a lizard swim in tight circles, each one curving as hard as it can. A short guide runs along the bottom; follow it together. It opens Skeleton in the rail on the left and picks Bones.

   You'll know it worked when: Each creature is a chain of dots: the snake's coils inside itself, the lizard's just closes, and the fish's stays a C.

2. Ask: all three swim in a tight circle, so why can't the fish make a ring? Then say the rule together: every dot follows the dot ahead and stays the same distance behind it. Nobody animates the tail; the rule, repeated down the chain, moves the whole body.

   You'll know it worked when: Your child can say the rule in their own words: follow the dot ahead and stay the same distance behind it.

3. Each joint can bend only so far. Open Body and find Flexibility: it runs from about 13 degrees per joint at the left to about 42 at the right. Work the fish together: 12 joints make 11 bends, each about 24 degrees, and 11 x 24 = 264 degrees.

   If you get stuck: If the answer is 12 x 24 = 288, the joints were counted instead of the bends. The bends are the gaps between the dots, so there is one fewer bend than joints.

   You'll know it worked when: Your child says 264 is less than the 360 degrees of a full circle, so the fish cannot close a ring however hard it turns.

4. Now predict before looking. The snake has 26 joints at about 29 degrees, and the lizard 16 joints at about 26. Your child works out each total and writes yes or no for a full ring, then checks in Bones.

   You'll know it worked when: The snake's 25 x 29 = 725 is about two circles, so it wraps around on itself. The lizard's 15 x 26 = 390 is just over one, so it just closes its ring.

5. Close the fish's ring. Select the fish (click it, or the 1 at the top), open Body and change one slider at a time: Flexibility to the right, or Length for more joints. After each change, count the dots in Bones, work out the new total, and check it.

   If you get stuck: If Length has disappeared from Body, Edit skeleton was pressed, which hides the simple sliders; press Reset to sliders. If the snake or the lizard changed instead, Body changes only the selected creature: click 1 at the top and check the name box says Fish.

   You'll know it worked when: The fish's chain closes into a ring: with Flexibility about seven tenths of the way along (11 x 33 = 363), or with about 16 joints (15 x 24 = 360).

6. The real question, without trying it: would a stiff snake still make a ring? Why?

   You'll know it worked when: Your child says yes, because a long body has many small bends, and many small bends add up past 360. That means they see that the angles add and that the rule repeats down the chain.

7. Finish with the big question: how can one small rule make a whole body move?

   You'll know it worked when: Your child says that one rule, follow the joint ahead and bend no more than so far, repeated for every joint, makes swimming, slithering and coiling.

Talk about it

Ask these while you work together. A child who has understood the lesson can answer them without help:

• What is the one rule every joint follows?
• Why does a fish with 12 joints have only 11 bends?
• How far can the whole fish bend? Is that a full circle?
• Which slider did you change, and what total did you work out?
• Without trying it: would a stiff snake still make a ring? Why?

Bring it into other subjects

• Science: real snakes have more than 200 vertebrae. Why can they coil so tightly? (Many small bends add up, just like the chain.)
• Math: a fish has 11 bends. What is the smallest bend per joint that makes a full circle? (360 / 11 = about 33 degrees.)
• Art: draw a creature as a chain of circles of different sizes, as the X-ray view in Skeleton shows them.
• Computer science: open Program the brain in Behave to see the program that steers the head.

Quick check

Name: ______________________________

1. A creature has 10 joints. How many bends does it have? (A) 10, (B) 9, (C) 11.

2. A creature has 20 bends, and each can bend 20 degrees. Can it curl into a full circle? (A) yes: 400 degrees is more than 360, (B) no: 40 degrees is too little, (C) no: 20 is less than 360.

3. A fish cannot close its ring. Name two different changes that could help, and say why each one works.

Answers

Measured in the app with the starter's three creatures circling as tightly as they can: the fish curls 269 degrees, the lizard 372, the snake 712. The worksheet's rounded degrees give 264, 390 and 725: close, because the slider's degrees are rounded to whole numbers.

Closing the fish's ring: Flexibility about seven tenths of the way along (about 33 degrees per joint: 11 x 33 = 363), or Length up to about 16 joints at the same flexibility (15 x 24 = 360), or a mix of both.