An object either floats or sinks in water depending on its density compared to water’s density, and the concept of buoyancy. If an object’s density is less than water’s density, it will float. If the object’s density is greater than water’s density, it will sink. Buoyancy, the upward force exerted by a fluid that opposes the weight of an immersed object, also plays a role. If the buoyant force is greater than or equal to the object’s weight, it will float; if the buoyant force is less than the object’s weight, it will sink.
Elaboration:
Density:
Density is a measure of how much mass is packed into a given volume. A material with higher density has more mass per unit volume than a material with lower density.
Buoyancy:
When an object is submerged in water, the water exerts an upward force on it, known as the buoyant force. This force is equal to the weight of the water displaced by the object.
Floating vs. Sinking:
An object floats if the buoyant force it experiences is greater than or equal to its weight. This means the object’s density is less than water’s density, and the object displaces enough water to support its weight. An object sinks if the buoyant force is less than its weight, meaning the object’s density is greater than water’s density and it cannot displace enough water to support its weight.
For a more helpful explanation for children, you can relate it to how tightly packed the atoms are in an object:
Sinking: Objects with tightly packed atoms are denser than water and sink.
Floating: Objects with loosely packed atoms are less dense than water and float.