Young Scientists
Float Market
A steel spoon sinks but a steel SHIP floats — because a boat floats by pushing water aside, and a denser fluid (saltwater, honey) floats more. That sets a weight CAPACITY. Now load the dock: pick which crates to ship to earn the most coins without sinking the boat. The richest crate isn't always worth the kilos it eats — it's a real packing puzzle (a knapsack). Solo head-to-head against four dockhand bots up to the perfect-packing Brine Baron, plus live whole-class rooms where every boat splashes to its waterline at once.
For Educators
Built for the classroom
Density and buoyancy through play — and the demolition of the most universal kid misconception, "heavy things sink." Re-implements the mechanic of PhET's Density/Buoyancy bench and the Exploratorium's "Will It Float?" tank: each round names a fluid and a target submerged fraction, and students BUILD an object by setting its mass (cargo) and volume (hull size — bigger hull traps more air), watching the live waterline. The deep idea, in the gated theorem: it is DENSITY (ρ = mass ÷ volume), not weight, that decides float vs sink — an object floats iff its density is below the fluid's — and the fraction left underwater is exactly density ÷ fluid-density (so ~90% of an iceberg hides). A hollow steel hull is mostly trapped air, so its average density drops below water — THE reason a steel ship floats. Our test proves floats() agrees with ρ_obj<ρ_fluid over 200,000 builds, the steel-ship flip crosses exactly at volume = mass/ρ_fluid, and — pinned, always run — a 10 kg object floats while a 0.5 kg object sinks in the same water: weight never decided.
Grades 3+ · four everyday fluids (water · oil · saltwater · honey) · solo is a best-of-5 Density Bench vs four builder bots — beat 🍯 Brine Baron (closest in 3+ rounds) for the License · live rooms are 3/5/7 rounds, every build splashing to its true waterline at the reveal.
Three ways to play
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Play solo
Five rounds against four builder bots. Each round, build to the float target. 🧊 Cube only ever changes how HEAVY its build is — so it can't hit the low targets, because the secret is changing the SPACE. Be closest in 3+ rounds to earn the License and unseal the Theorem.
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Host a Room
One fluid + target for the whole class on the projector; everyone builds secretly. At the reveal every build drops in and splashes to its real waterline at once — let them notice the heaviest one isn't the winner.
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Join on any device
Set your cargo and hull size, watch your waterline, and hit the target. Closest in the room wins the round.
Run a live class in 5 steps
- Open the Room — pick the rounds; share the 4-letter code / QR / link.
- Students join on any device; everyone gets the same fluid + float target.
- Each kid builds an object (cargo + hull) to hit the target — builds stay secret until the reveal.
- At the reveal, every build splashes to its true waterline together. Ask: did the heaviest one sink?
- Reveal the rule (density, not weight) only after they've felt it — then send the curious to the Dojo's steel-ship Tank Lab and the theorem.
Tip: host on a laptop or projector (the big screen shows the code, QR and leaderboard); students join on their phones or laptops.
