Easy science experiment: deep sea diver in a bottle

Did you know that sharks are never still in the water? They swim all the time! This is because they are denser than the water and if they stopped swimming they would sink!

Whether something floats or sinks depends on its density. It’s how we measure how heavy an object is for it’s size. A steel cube, for example, is a lot heavier than an ice cube that’s the same size. If you put them both in water, the steel cube will sink while the ice cube will float. Ice is less dense than water, in other words an ice cube weighs less than a cube of water of the same size.

This deep sea diver experiment is the perfect way to demonstrate the basics of density while having a whole lot of fun too!

You’ll need :

  • A two litre plastic bottle with lid
  • A bendy straw
  • A paper clip
  • A foil tin / clean takeaway container
  • Blue tack
  • Scissors
  • A bowl of water

bottle diver items

Making Your Deep Sea Diver

1. Cut out the shape of your diver from the pie tin, making him tall and thin, about 7 cm by 2 cm so he can fit through the neck of the bottle. Make him simple, just a head , a body and a pair of feet will do.

aluminium diver

2. Bend the straw double, then cut it so that you have a U-shaped piece about 2.5 cm long.

bendy straw

3. Slide the open ends of the straw on to the two ends of the paper clip.

paper clip straw

4. Gently slide the paper clip and straw between the diver’s legs and up on to his body. The straw should be on his back, bent at the top behind his head, looking like a real diver’s air tanks. Make diving boots out of blue tack and put them on his feet.

diver with straw and blue tack boots

5. Try floating your diver in a bowl of water. Carefully adjust the amount of blue tack on his boots until he just floats.

Fill the bottle with water and put the diver inside. Make sure the bottle is full to overflowing then screw the lid on tightly. The diver should float to the top – although ours also sunk gloriously.

diver floating in bottle


If you squeeze the bottle the diver will sink to the bottom and if you let go, he’ll float back up. You can make him float at any depth you like.

The science bit: how density works

When you squeeze the bottle, water is pushed into the straw. This compresses the air in the straw, making the diver heavier. His density increases so he sinks.

When you let go, the pressure of the air trapped in the straw pushes the water back out. This means the diver is now less dense than water, so he floats back up.

So… what makes a metal boat less dense than the water?

Well, for more fun you can test different objects (like solid objects made of glass, metal, wood, plastic, modelling clay, aluminium foil) to see which float of sink. For example if you flatten the aluminium foil or the modelling clay into a bowl shape, they will float.

Small, heavy things like coins and stones sink whereas large, light things like corks, float. When you make a big, hollow boat shape out of something small and heavy like modelling clay, most of the boat is actually filled with air. Together, the boat and the air inside are less dense than the water so the bloat floats.

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