CMSC · 2026 · Padova, Italy

Building Joyful
Interactives

Neeldhara MisraIIT Gandhinagar · interactives.neeldhara.com

Do not introduce the talk yet. Pick up the four physical cards. "Before I tell you what this talk is about, I need a volunteer and about ninety seconds." Hand the volunteer four cards, face down. Ask them to look at the bottom card and remember it. Do not look. (Timing target: you should be at the trick slide within 30 seconds.) PACING (45 min). Protect Trust: it gets 8 minutes no matter what. 0-2 title; Gather Town (plugging in CS Unplugged) 2-8 "watch the world learn", textbook, VisuAlgo, Cortesi's porridge (pause per sentence), static bubble sort, the two tweets 8-12 3blue1brown colliding blocks, quaternions, Ben Eater's comment, the Anatomy skill clip 12-22 Baby Hummer: perform, then walk all four panels on the one slide 22-30 the piano, in full 30-40 Trust, live (8 min, untouchable), then its lesson 40-45 thanks, questions (The rest of the talk is being reworked: slack here is deliberate.)

Gather: video chat in fully customizable 2D interactive spaces

Gather Town - my story of plugging in CS Unplugged

Nicky Case

A typical worked out example from a typical textbook

1.4 MB screen recording attached in the thread: save as cmsc/media/visualgo.mp4

Examples made interactive (visualgo.net)

…it is my measured opinion that this animation has all the explanatory power of a glob of porridge flung against a wall.

To see why I say this, try to find rough answers to the following set of simple questions with reference to it:

After what percentage of time is half of the array sorted?
Can you find an element that moved about half the length of the array to reach its final destination?
What percentage of the array was sorted after 80% of the sorting process? How about 20%?
Does the number of sorted elements grow linearly or non-linearly with time (i.e. logarithmically or exponentially)?

— Aldo Cortesi

Bubble sort, drawn statically (Aldo Cortesi)

A second tweet critical of the static bubble sort image

A tweet critical of the static bubble sort image

video attached in the thread: save as cmsc/media/3b1b-colliding-blocks.mp4

Why colliding blocks compute pi by 3blue1brown

Visualizing quaternions

video attached in the thread: save as cmsc/media/anatomy.mp4

The Anatomy Skill

You walk through all four panels on this one slide: click the tabs at the top of the figure (1 Perform, 2 S and X in tandem, 3 The other two, 4 Finale decoded). Arrow keys stay with the deck; Esc in the figure returns focus. PANEL 1, PERFORM. Do it physically first with the volunteer, then on screen. Begin: top card to bottom, turn over the new top card. Cut and turn two as often as they like ("you are in control"). Finale: turn top card over and put it under; next card under; turn the top card over. Spread: one card faces the other way; it is theirs. Credit: Baby Hummer, Charles Hudson, after Bob Hummer; opening trick of "Magical Math" on grokabook. PANEL 2, S AND X IN TANDEM (ring, badges, buttons only). S = the special card (volunteer's, originally bottom; orange badge). X = the tracker (the card Begin turns face up). Top of deck at twelve o'clock, bottom at nine; a cut just turns the ring. After Begin, S is ACROSS from X, facing the other way. Plant "maybe S always stays across?", then cut 1 + Cut & turn two: S lands NEXT TO X, facing the SAME way. Why: a cut changes nothing; turn two with both or neither of S, X in the top two leaves the pair alone; with exactly one, that card moves one step AND turns over. So S is always "across, other way" or "next to, same way". PANEL 3, THE OTHER TWO. Right: the only three pictures a legal deck can show. Read clockwise from X, drawn with X face up (if the real X is face down, the real deck is the same picture turned over). The live position's picture is outlined; no Finale here, it is panel 4. Where S sits FORCES everything else: S on X's left (nine o'clock): X up, other two on top of the ring opposite ways (three o'clock down, six o'clock up), S up. S across from X: X up, the other two on X's left and right, both down, S down. S on X's right (three o'clock): X up, S up, the other two opposite ways (six o'clock up, nine o'clock down). So: S next to X => the other two are neighbours facing opposite ways. S across => the other two are across from each other facing the same way. The ringed card: the number of face-up cards starts at 1 and every turn-two flips two cards, so it stays odd. Exactly one card faces the minority way. S left: the unmarked card at three o'clock. Across: X. S right: the unmarked card at nine o'clock. The finale's job (next panel): afterwards S is the unique minority-facing card. No card turns into another; the ROLE of odd one out moves to S. (Checked while building: all 48 reachable positions are one of these three pictures, 16 each; the finale makes S the odd one out in all 48.) PANEL 4, FINALE DECODED. Four clicks. Net effect: everyone moves two places, the cards that started 1st and 3rd are turned over. Across pairs: both or neither turned. Neighbours: exactly one turned.

One piano key, one problem at a time

open full screen ↗ · press and hold the key (or Space) · arrows step

One connection at a time

storyThe 1914 Christmas truce: why could enemies trust each other?
thereforeA one-shot game: cheating wins.
butThe trenches face the same enemy every day.
thereforeThe repeated game, then a tournament, then evolution.
butPeople make mistakes. Therefore: noise, and forgiveness.

Nicky, at the NYU Game Center (2019): working memory holds about four chunks, so give learners one new connection at a time.

Roast the vegetables.

Don’t cover broccoli in chocolate. Bring out the flavour already in the idea.

Thanks: Nicky Case (Evolution of Trust, CC0; two talks), Mark Rober (piano), Charles Hudson & Bob Hummer (the trick), Ryan’s Anatomy, Making Software.