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There is a reason why some objects float in water and some do not. The ancient Greek genius Archimedes figured it out. As legend has it, he discovered the principle of buoyancy one day as he lowered himself into the bathwater. He jumped out of the bath and went running through the streets yelling, "Eureka!" which means "I found it!"

What Archimedes figured out is that an object in a liquid is buoyed up (pushed up) by a force equal to the weight of the liquid the object displaces (pushes aside). Today, this is known as Archimedes’ principle.

To test this principle, let’s imagine that we have four toy ducks all the same size. Because each is made of different material, they are not the same weight. The water duck is included to show that in this example 0.45 kg (1 lb) of water would occupy the volume of each of these ducks..

 Imagine that any of these ducks is pushed completely underwater. Its volume displaces (pushes aside) 0.45 kg (1 lb) of water, an amount equal to that for the duck made of water. This displacement produces an upward buoyant force of 0.45 kg on the submerged duck.

Now imagine releasing these ducks one at a time on top of the water. Any duck that weighs less than a pound will sink down into the water until it displaces enough water equal to its weight. Then the upward buoyant force will equal the downward weight of the duck and it will float.

For example, the rubber duck, with a weight of 0.057 kg, will sink into the water until it displaces 0.057 kg of water, and then it will float. When the rubber duck is floating, the upward buoyant force of 0.057 kg exactly balances the duck’s downward 0.057-kg weight. Similarly, the 0.22-kg wood duck will sink deeper into the water until it displaces 0.22 kg of water, and then it will float.

If the water duck is released on top of the water, it will sink until completely submerged. At that point, it will be displacing exactly its own weight in water, so the upward 0.45-kg buoyant force will balance the downward duck weight of 0.45 kg. If the water duck is completely submerged and released at any level underwater, it will remain at that level.

The 3.63-kg steel duck, on the other hand, will displace only 0.45 kg of water when completely submerged, producing a 0.45-kg upward buoyant force. This is not enough to hold up the 3.63-kg weight of the steel duck, so it sinks to the bottom.