Floating and Sinking
When we put an object in water, it will either float (stay on top) or sink (go down). Whether it floats or sinks depends on its density.
What Is Density?
Density is how heavy an object is for its size. An object that is light for its size has low density and floats. An object that is heavy for its size has high density and sinks. A cork floats because it is light for its size. A stone sinks because it is heavy for its size.
Key idea / 要点
An object floats if it is light for its size (low density). It sinks if it is heavy for its size (high density).
Example / 例子
A small stone sinks, but a big wooden log floats. The log is much bigger yet still floats because it is light for its size.