Why do some things float but not others?
Things that are less dense that water float while things that
are more dense sink.
An object's density has to be less than what it's floating in.
If it's less, the object floats. If it's more, the object
For more information about buoyancy see the related questions
There is an exception to the above density proposal.
Sometime a solid steel object which is more dense than water can
"float" on the surface of water.
Solid steel objects are able to "float" on the surface of water
due to the surface tension of the water.
Small solid steel objects such as double edged razor blades,
needles and even paper clips can float on this surface tension film
where water meets with the air above.
The above objects have to be carefully lowered onto the surface
of water and they will "float".
To prove they are more dense than water, simply push them under
the surface and they will sink to the bottom.
See the related links to see a floating needle and floating