How is it that an extremely heavy ship made of metal floats on water? But a tiny piece of metal like a spanner easily sinks into it. Let's understand this through an activity. I have a small piece of metal and this metal easily sinks in water. And I also have a steel container which floats. Also if I place this small piece of metal, if I place this metal in this steel container, then it floats. Why does the steel container float and why does the metal sink? Let's look at the base of the container. Notice there is no water inside the container. So if I only look at the base of the container, this much amount of water must have been displaced. If there is displaced water, there must be buoyant force. We know that the buoyant force equals the weight of the displaced liquid. That is water in this case. So the reason why this steel container floats is that the buoyant force must be more than the weight of the steel container. If we look at the metal piece, then there is some displaced water again. But the reason why this sinks is that the weight of the displaced liquid is not much. So the buoyant force is small and must be less than the weight of the metal piece. Let's come back to our original question. Why do ships float? Again, let's focus on the base of the ship. There is no water inside the ship, right? That means this much amount of water must have been displaced. And since this is a lot of water, it has a lot of weight. And therefore the buoyant force acting on the ship must be very large. Now let's say we take the same amount of metal and flatten it. Now this will sink, right? But why? Well, because now you see it is only displacing this much amount of water. It's no longer displacing the water on top of it because the shape has changed, right? And since the displaced water has little weight, so the buoyant force acting on the same piece of metal is very less. So that's the secret. Ships have a lot of empty space, just like the steel container in the beginning, such that their metal occupies a large volume underwater. Because of which they will displace a lot of water, making sure that the buoyant force is large enough to support its weight. So to summarize, an object will float if the buoyant force is greater than the weight of the object and it will sink when the buoyant force is smaller than the weight of the object.