Guess what floats and what sinks, then drop it in and see. Load a boat until it goes under, squish clay into a boat that floats, add salt until an egg rises, and pilot a submarine.
The big ideas
What the levels teach, in grown-up words.
It is about how heavy something is for its size. Scientists call that density. Anything lighter than the same amount of water floats; anything heavier sinks. Big or small on its own does not decide.
Water pushes up on everything in it. The push equals the weight of the water that got shoved out of the way. A floating thing sinks in just far enough for that push to match its own weight, which is why a loaded boat sits lower.
Shape changes how much water gets pushed away. A ball of clay sinks, but the same clay shaped like a bowl floats and can carry marbles. Steel ships work the same way: the hull is mostly air.
Salty water pushes harder. Dissolved salt makes water heavier for its size, so an egg that sinks in fresh water floats in salty water, and people float easily in the sea.
Submarines and fish change their own density. A submarine floods its tanks with water to dive and blows air back in to rise. Many fish adjust a bag of gas inside them, the swim bladder, to hover at a depth.
Try it at home
Quick experiments that match the game.
Bath-time guessing game. Collect ten safe things from around the house. Guess float or sink for each, then test them in the bath or a bowl. Which guesses surprised you?
Foil boats. Make a boat from a square of kitchen foil and count how many coins it carries before it sinks. Then make a wider one and count again.
The floating egg. Put an egg in a glass of water (it sinks). Stir in salt a spoonful at a time until it floats. Pour fresh water gently on top and it will hang in the middle.
Orange with and without its coat. Float an orange, then peel it and try again. Ask why the peel might make a difference.
Ask after playing:
Why did the big pumpkin float but the little marble sink?
What happened to the boat each time we added a crate?