1. The shape of an object can help it to float or cause it to sink.
Try this
Put a lump of Plasticine the size of a golf ball into a bowl of water and watch what happens.
Click 1
Now make the Plasticine into the shape of a boat and watch what happens this time.
Click 2
The boat-shaped Plasticine pushes down on a bigger bit of water. The water can push up more of the Plasticine and hold it up so the boat floats.
The lump of Plasticine pushes down on a small bit of water so it sinks.
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