Why some objects sink and others float is puzzling at first. Sometimes big objects float and little objects sink! 

Sinking and floating really have nothing to do with the size of an object. It all depends on the density. Density is how loosely or tightly packed molecules are in an object. Molecules are the tiny particles that make up all things, they can be seen with a microscope. The tighter these molecules are packed together, the denser the object will be and it will sink. 

A stone, a paper clip, and a coin all have very tightly packed molecules. 

Cork, a wooden branch, and a sponge have loosely packed molecules which is why they float, they are buoyant. Buoyancy is the name given to something that can float in water.

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Gravity plays an important role in whether an object sinks or floats. Gravity is a very strong force that pulls objects down. Here’s an easy experiment you can try. Place an ice cube in a glass of water. Do you notice how the ice cube floats? Part of it is under the water and part of it is above the water. Gravity is trying to pull the ice cube down but the density of the ice cube is so low it is pulling it up. It’s a bit like a tug of war. The force pulling the ice cube down is similar to the force pulling it up, so in the end, the ice cube stays somewhere in the middle.

Now let’s look at something really crazy. How do big ships float in water? Ships are dense and heavy, gravity pulls them down, so why don’t they sink? Ships float because their hulls are filled with air, this makes them less dense. Just like ice cubes, ships are both underwater and above water!

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Fun facts

  • Ships float higher in denser waters. Salt makes seawater denser, making ships float higher in seawater than in freshwater.
  • Ships float higher in cold seas than in tropical ones. They float higher in winter.
  • It is easier to lift a heavy person in water than on land. 
  • The shape of objects can determine if it sinks or floats. A ball of clay will sink but if you flatten the clay into the shape of a boat, it floats!
  • Objects filled with air float.
  • Different liquids have different densities. If you mix oil, water and corn syrup, the oil will float to the top because it is the lightest, the water will sit in the middle and the corn syrup will sink because it is the densest.