How Does Things Float?

Things float because they displace a volume of fluid that is greater than their mass.  A very simple way of looking at this is to say that anything that floats weighs the same as the water it pushes out of the way.

Here is a link to a longer explanation - www.newton.dep.anl.gov

For something to float, its density must be less than the density of that in which it is floating.

For example a ship is built of very dense material such as steel which has alot of mass, however, there is a great deal of space within the ship full of things much less dense than water such as air. So the entire ship's density overall is less dense than water so it will float.

Objects float when they displace more water than what they actually weigh.

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