Why does density decide whether something floats or sinks? 
Possible Duplicate:
Balloons and lifting gases 

I know that the density of an object dictates whether or not it will float, but why?
My idea is: an object with a lower density has a spread out mass distribution. Since water's density is 1 g/cm$^3$, when you have an object with density lower than water's, it exerts a force on the water due to gravity, but because at every point the weight of the object is less than that of water, it is unable to push the water away.
I know this doesn't make any sense, please help!
 A: The answer hinges on Archimedes' principle wiki. If the object is denser than water it is more massive than the water that it displaces. This means that the object experiences greater gravitational force than the water and so sinks.
A: I agree with hrandjet but I think the best way to "get a feel" for what's going on is to think of the floating object and the medium in which it floats in a competition.  Both the object and the liquid "want" to get to the lowest point possible (the location with the lowest potential energy).
If you think of a microscopic object less dense than water, it is easy to imagine that a small drop of water is more dense and thus "feels" a greater force per unit surface area.  If our microscopic object were placed in water, successive drops of water would slip around the object and the object would rise to the surface and float.  When an object like a boat floats, the same argument holds except that each individual little microscopic pieces of the hull float and they are all connected together.  The argument doesn't strictly apply since the microscopic pieces of the hull are individually more dense than water, but the pseudo-density, if you will, is calculated from the density of the entire ship. This is because objects less dense than water will float 
