Classroom · Psychology
Two equal lines. How far is the room fooled?
Each student guesses how far off they will be, then matches two lines on their phone by eye, with no number to read. Every match lands on a shared ruler on the projector. Told that the lines were equal, the room matches again, and then once more with no fins at all.
- Up to 600 people
A practice run with fictional students. No real classroom or join code is created.
Try an answer on the phone. Lead the lesson from the screen.
On their phone
You are presenting
Setting up the room…
Lesson deck
Teach it as a ready-made lesson
Hook, how it works, the live activity and takeaways. Edit anything.
The room's ruler
A ready-to-teach lesson deck: a hook, how it works, the live activity and takeaways for the debrief.
6 slides · 1 activity
What it teaches
Students measure a visual illusion on their own class. They see that its error is systematic rather than random, because the two versions miss in opposite directions; that knowing the lines are equal does not remove it; and that with no fins the same hands land close to the truth.
- Visual perception
- Systematic bias
Debrief, while the boards are still up
- 01
Did group A and group B miss in opposite directions? What would that tell us about whether this was carelessness?
- 02
You were told the lines were equal. If that didn’t fix your second try, why not, and what does it say about where the illusion happens?
- 03
With no fins, how close did the room come to Equal? So what are the fins doing?
- 04
You predicted your error before you started. How did your prediction compare with what happened, and why are we bad at predicting our own perception?
- 05
Where might the same kind of error matter outside a classroom: reading a gauge, judging a gap when driving, or a chart designed to mislead?
How it works
- 01
Add it to your library
Sign up free and pull the activity from Decka's built-in classroom catalog — one click copies it into your own library, ready to run from Decka or inside PowerPoint.
- 02
Students join by QR
Everyone scans from the projector and plays in their phone's browser. No app, no account, nothing to install.
- 03
You run the room
You move the activity phase by phase: collection closes, the boards resolve on the big screen, and the debrief happens while the results are still up.
An optical illusions game that measures the room
Each phone draws the Müller-Lyer figure: a top line with fins and a plain line under it. The student drags the end of the bottom line, or steps it shorter or longer, until the two look the same length, then taps “They look equal”. No number is shown or read aloud while they match, so the eye is the only instrument. By order of joining, half the room sees fins pointing out and the other half fins pointing in, and neither side is named on a phone.
Before anyone matches, everyone guesses how many percent off they will be; the guesses stay sealed until the first match lands. When matching closes, every match lands as a tick on the room’s ruler, side A on one rule and side B on the other, beside Equal. Noise scatters both ways; an illusion pushes everyone who saw the same fins the same way, so two sides that miss in opposite directions were fooled by the figure, not by careless thumbs. The room’s predictions then land beside what it actually did.
Then the big screen shows what Equal looks like, both lines exactly the same length, and the room matches again, knowing. The told ticks hang under the same rule as the first ones, and each person’s two matches are set against each other: closer to Equal, about the same, or further. The illusion usually survives the telling, told and still fooled, because what you know does not change what you see; psychologists call this cognitive impenetrability. A last match with two plain lines shows how precise the same hands are, with the middle half of the room’s matches marked beside Equal.
A student who cannot judge lines on their screen can say so and is counted under “Could not judge”, never as a zero; a phone that sends nothing is counted as standing by. The projector shows counts and the shape of the room’s matches, never names, and each phone shows its owner’s own matches alone. Try the screen-and-phone preview, where the phone is yours to match with and fictional classmates match through the same runtime as the lesson; the classmates are illustrative, not a real class.
FAQ
Optical illusions game questions, answered
What do I need before class?
No equipment. Phones with screens wider than 320 px, so the two lines are large enough to judge, and a projector or big screen for the room’s ruler. Every phone draws its own figure, so there is nothing to print or hand out.
Why is the number hidden while students match?
The lesson measures what the eye sees. A number on the screen would let a student match the number instead of the lines, so the phone shows only the figure and the line being moved, and announces only “Longer” or “Shorter”.
Is this a test of intelligence or eyesight?
No. Everyone’s eyes do this; it is not a test of intelligence. The illusion is a property of how human vision works, and the board shows the room’s spread and its middle, never a name. Nobody is deceived: each side’s fins are drawn on the big screen at the first reveal and explained in the debrief.
What if the room is barely fooled?
Then the board says so in plain numbers, and the first debrief question becomes why this room might be resistant. How strongly people see the illusion varies between people and between groups, and the lesson reports what your class actually did.
What if being told fixes the second match?
Then the board shows it: the told ticks move toward Equal and the paired counts show how many got closer. Ask what students did differently. Some over-correct by deliberately setting the line short, which lands them on the other side of Equal.
What if many students cannot judge their screen?
Each match offers “I can’t judge lines on this screen” instead of a value. Those students are counted apart under “Could not judge” on every board, never folded in as a zero or a match, and each can undo it and match until matching closes.
How long does it take?
A guess and three matches, each on its own clock, each followed by a board you move on from when the room is ready, then the debrief. Every clock is a setting you can change before you run it.
Do students need an app or an account?
No. Students scan a QR code from the projector (or open a link) and play in their phone's browser. Nothing to install and nothing to sign up for.
Does it work in PowerPoint or Google Slides?
In PowerPoint, yes: open the Decka add-in on a slide and choose the Classroom tile. Google Slides is not supported yet; there you present from Decka itself. Students play from their phones either way.
How many students can play at once?
Up to 50 participants per live activity on the free plan, 150 on Pro, and 300 on Badass.
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