Sink or Float Dock

Little levels (ages 4 to 6)

1. Will it float? Guess for an apple, a rock, a duck, a coin and a cork, then drop each one in.
2. Big or small? A big tennis ball and a little golf ball: tap the one that sinks.
3. Sort the dock Put six things in the Floats box or the Sinks box, then drop them all in.
4. Load the boat Guess how many crates the boat can carry, then load it until it sinks.
5. Make it float A ball of clay sinks. Squish it into a bowl and it floats.

Big levels (ages 7 to 9)

1. Same size, different weight Wood, wax, steel and ice cubes, all one size, against a cube of water.
2. Peel the orange An orange floats. Peel it, and it sinks.
3. Shape the clay Ball, pancake, cup or bowl, then count how many marbles the floating shape carries.
4. Cargo captain Load crates that weigh 1, 2 or 3 up to the load line, then set sail.
5. Salty sea An egg sinks in fresh water. Add salt a spoon at a time until it rises.
6. Submarine Fill the tanks with water to dive, half and half to hover, air to come up.

The big ideas

• 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

1. 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?
2. 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.
3. 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.
4. 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?
• How did the egg float after we added salt?