The Cup Trick
Four cups. Which one holds the most water? Can you tell?
Concept
"Tall isn't always more. Wide isn't always more." Kid discovers that shape can trick your eyes about volume — three cups filled to the same level (skinny, medium, wide) can hold very different amounts, and only counting tells the truth.
The Experience
Kid sees three differently shaped cups — tall and thin, medium, short and wide — each filled with juice to the same visible level, and guesses which one has the most. Almost everyone picks the tall one.
Pouring each cup into identical measuring beakers reveals the truth: the wide cup held the most juice, and the tall cup that looked fullest actually held the least.
The pattern repeats with new cup shapes, and the guess keeps getting fooled — until a "Block Count" tool arrives: each cup's interior is mapped onto a grid of uniform blocks, and multiplying rows × columns gives the actual amount of juice, no guessing required.
After using the block-count tool on a couple more pairs, the kid starts getting it right every time — not because they're guessing better, but because they're calculating.
The final challenge removes the pouring altogether: rank three new cups from most to least juice using only the block-count tool and multiplication — and get it right without a single pour.
What Math Does The Kid Do?
- Estimates volume visually and discovers how unreliable that estimate is.
- Counts rows and columns in a block grid.
- Multiplies rows × columns to calculate volume in blocks.
- Compares volumes across differently shaped containers.
- Replaces guessing with systematic counting.
Why This Matters
Shape won't trick your eyes anymore. You've seen a tall glass hold less than a short, wide one — so from now on, you count the volume instead of guessing by height.
Frequently asked questions
The Cup Trick is tagged for Grades K-2.
No — The Cup Trick plays directly in the browser, right on this page. No login, no download.
Fact-check a real-world claim — is it true, or does it just look true?
No — The Cup Trick is free to play, no sign-up needed.