Break the plasticine into two balls that are the same size.
Shape one ball into a boat. It will depend on the size of your plasticine but you can make your boat about the same size as your hand when it is cupped. Make sure all the sides are about the same height.
Place the plasticine boat and the ball on the surface of the water.
When you placed both objects on the surface of the water the results were very different, even though both pieces of plasticine have the same weight. The ball takes up a smaller space than the boat. The amount of water pushed aside by an object equals the force of water pushing upward on the object. The larger plasticine-boat pushes more water out of the way than the ball and creates enough upward force to cause it to float.
Whether or not a given object will sink or float in a fluid is determined by the buoyant force on the object. This was first observed by Archimedes. His observation has been formalised into Archimedes Principle, which states: "An object partially or wholly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."
Archimedes reasoned that the buoyant force on an object must be equal to the weight of fluid that object displaces. If the weight of an object is greater than the weight of displaced fluid, it will sink. If the weight of the object is less than the weight of displaced fluid, it will rise.
Objects that sink are frequently termed negatively buoyant. Objects that float are termed positively buoyant. Objects that stay stationary at depth are said to be neutrally buoyant. You can test this with different types of wood formed into blocks of the same size.
When you place a block of wood in a bucket of water, the block displaces some of the water, and the water level goes up. If you could weigh the water that the wood displaces, you would find that its weight equals the weight of the wood.
A block of wood made of hardwood sits deeper in the water (and therefore displaces more of the water) than does a block of pine wood. The reason is that it's heavier for its size, or denser. That means the molecules that make it up are more closely packed together than the molecules that make up the pine.
If you could somehow keep increasing the density of the block, it would sink lower and lower into the water. When its density increased enough to displace an amount of water whose weight was equal to the weight of the block, it would, in a sense, become weightless in the water.
A ship will float as long as it weighs less than the water it pushes out of way, or displaces. Ships can use materials in their hulls that are heavier than water, but there must be air within the ship. Since the air doesn't weigh as much as the water, this lowers the weight of the ship compared to the same volume of water.
Break the plasticine into two balls of even size. Shape one ball into a small boat.
A ball of plasticine, shaped like a boat, will float even though it has the same weight as the ball that sinks.