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Why do things float or sink?

Anchor under waterWhy do some things float and other things sink? It depends on the density of the object. Density is a measure of how packed together an object’s matter (atoms and molecules) is. The more matter it contains, the denser it is. If it is less dense than water, it floats, but if it is more dense, it sinks.

Floating in water

A cork is less dense than water, but a stone the same size is denser. The cork floats but the stone sinks. Rubber rings float because they are full of air—which is less dense than water. People are almost the same density as water, which is why you just about float. Air-filled buoys are attached by a chain to a heavy weight on the sea bed that stops the buoy floating away.

Gold is nearly 20 times the density of water. But it is not the densest element. That is osmium.

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